Additional Proxy Soliciting Materials (Form DEFA14A)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934 (Amendment No. )

Filed by the Registrant ☒

Filed by a party other than the Registrant ☐

Check the appropriate box:

☐ Preliminary Proxy Statement

☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☐ Definitive Proxy Statement

☒ Definitive Additional Materials

☐ Soliciting Material Pursuant to §240.14a-12

ADTRAN Holdings, Inc.
(Name of Registrant as Specified In Its Charter)

Not applicable.

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check all boxes that apply):

☒ No fee required

☐ Fee paid previously with preliminary materials

☐ Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11

May 1, 2026

RE: ADTRAN Holdings, Inc. ("Adtran" or the "Company")

2026 Annual Meeting of Stockholders - May 13, 2026

Proposal 1 (Election of Directors)

Dear Adtran Stockholders,

On behalf of the Company's Board of Directors, I am writing to ask for your support by voting in accordance with the recommendations of our Board of Directors on all of the proposals included in our proxy statement, which was filed with the U.S. Securities and Exchange Commission (the "SEC") on March 27, 2026 and is available at both https://materials.proxyvote.com/00486H and the "SEC Filings" section of our website at https://investors.adtran.com (the "Proxy Statement").

We are requesting your support on Proposal 1 - the Election of Directors, and we specifically request you vote "FOR" H. Fenwick Huss, Gregory J. McCray, Jacqueline H. Rice, and Nikos Theodosopoulos in addition to the other director nominees.

It has come to our attention that Institutional Shareholder Services ("ISS") has recommended that stockholders vote against the re-election of the members of the Audit Committee - H. Fenwick Huss, Gregory J. McCray, Jacqueline H. Rice, and Nikos Theodosopoulos, with ISS citing the persistence of material weaknesses in the Company's internal control over financial reporting over multiple years.

We understand why this issue has been raised, and we take it seriously. Below, we want to share the facts about what our Audit Committee has accomplished, what has changed, and why we believe continued oversight by these directors is in the best interest of our stockholders: (1) we have taken remedial actions to address our outstanding material weaknesses and continue to test the related controls that we have already implemented; (2) we timely filed our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, which included the report of our independent registered public accounting firm; (3) we have made significant organizational investments to enhance our control environment; (4) Glass Lewis has recommended that stockholders vote "FOR" the re-election of the members of the Audit Committee; and (5) our experienced Audit Committee provides significant and effective oversight over the financial reporting process at the Company, and continued oversight of the Company's remediation efforts by the Audit Committee members who are closely familiar with the material weaknesses is critical.

1. We have taken remedial actions to address our outstanding material weaknesses and continue to operate and test the related controls that we have already implemented

The Company has taken remedial actions to address the material weaknesses in its internal control over financial reporting that are currently outstanding. These material weaknesses include:

To remediate such material weaknesses, the Company completed the following activities during the second half of 2025:

Based on testing performed, management has determined these enhanced and additional controls have continued to operate effectively through the first quarter of 2026. While these material weaknesses cannot be considered remediated until the applicable controls have operated for a sufficient period of time and continue to be successfully tested, management believes that the continued operation of the activities outlined above in subsequent reporting periods will be effective in remediating such material weaknesses.

2. We timely filed our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, which included the report of our independent registered public accounting firm

The Company's Annual Report on Form 10-K for the fiscal year ended December 31, 2025 was timely filed, and the consolidated financial statements and internal control over financial reporting were audited by the Company's independent registered public accounting firm, as evidenced by that firm's report included therein. For additional details, see Part II, Item 8 of the Company's Annual Report on Form 10-K for the fiscal year ended December 31, 2025, filed with the SEC on February 26, 2026.

3. We have made significant organizational investments to enhance our control environment

Over the course of the past 15 months, in addition to the remediation efforts described above, the Company introduced new leadership to its accounting, financial reporting and internal audit teams, which we believe has contributed to an enhanced control environment. Among other enhancements, this includes the hiring of the individuals serving in the below listed roles. These efforts were overseen by the Audit Committee.

These appointments reflect the rigor, seriousness, and commitment with which management and our Audit Committee are addressing our control environment.

4. Glass Lewis has recommended that stockholders vote "FOR" the re-election of the members of the Audit Committee

Glass Lewis has recommended that stockholders vote "FOR" the re-election of each of the members of the Audit Committee. Specifically, Glass Lewis noted the following: "In its most recent annual report, the Company discloses that its internal control over financial reporting are not effective as of December 31, 2025 as a result of material weaknesses that remain in its accounting controls and procedures. The Company has, however, remediated several previously disclosed material weaknesses. Further, the Company has provided stockholders with an updated remediation plan that clearly outlines the Company's progress toward remediating the material weaknesses." Glass Lewis further stated, "The audit committee has demonstrated responsiveness to this issue and has outlined a credible remediation plan."

5. Our experienced Audit Committee provides significant and effective oversight over the financial reporting process at the Company, and continued oversight of the Company's remediation efforts by the Audit Committee members who are closely familiar with the material weaknesses is critical

The Audit Committee is responsible for, among other items, fulfilling its oversight responsibilities with respect to: (i) the Company's financial reports and other financial information provided by the Company to the public or filed with the SEC; (ii) the Company's compliance with legal and regulatory requirements; (iii) the Company's systems of

internal control regarding finance, accounting and legal compliance that have been established by management and the Board of Directors; (iv) the qualifications, independence, and performance of the Company's independent auditors; (v) the performance of the Company's internal audit function and independent auditors; and (vi) the Company's auditing, accounting, and financial reporting processes generally. Our Audit Committee has provided critical oversight of management's remediation efforts described above. From the time each material weakness was identified, the Audit Committee has been regularly apprised of and has actively overseen management's remediation priorities, set expectations for pace and rigor, and maintained regular oversight of testing results and remediation efforts, with regular substantive communications and discussions with management, internal audit and the Company's independent auditors.

H. Fenwick Huss, Gregory J. McCray, Jacqueline H. Rice, and Nikos Theodosopoulos bring diverse thinking, financial expertise and critical sets of experiences to the Board of Directors and Audit Committee as follows:

During the fiscal years ended December 31, 2025 and 2024, the Audit Committee met eight (8) times and eleven (11) times, respectively. At least quarterly, the Audit Committee reviewed the status of internal control testing and remediation of the material weaknesses. During each of these meetings, the Audit Committee met in private sessions with the Company's auditors, as well as with our Chief Financial Officer. These private sessions included further discussion of the status of the remediation of the material weaknesses each quarter. Furthermore, the Audit Committee ensured that the Company was dedicating sufficient funds to the remediation of the material weaknesses. With the Audit Committee's oversight, the Company has made significant progress in improving its internal control environment, including the remediation of certain material weaknesses as of December 31, 2025. We believe that the continued operation of the remediation activities that have been implemented to date will be effective in remediating the remaining material weaknesses. Such material weaknesses cannot be considered remediated until the applicable controls have operated for a sufficient period of time and management has concluded that, through testing, these controls are operating effectively. Continued oversight of the remediation of the remaining material weaknesses by the Audit Committee, who are the members of our Board of Directors most closely familiar with the Company's internal control environment and remediation efforts, is critical. We believe the care and attention that our Audit Committee has given and will continue to give to this remediation process will contribute to a stronger internal control environment in the future. The Company has

developed significant momentum under their oversight during the past year. Therefore, electing H. Fenwick Huss, Gregory J. McCray, Jacqueline H. Rice, and Nikos Theodosopoulos is in the interests of both the Company and our stockholders.

The Company remains committed to maintaining a robust internal control environment and will continue to evaluate and enhance its controls as necessary to support the integrity of its financial reporting.

We consequently urge you to vote FOR H. Fenwick Huss, Gregory J. McCray, Jacqueline H. Rice, and Nikos Theodosopoulos, as well as the other director candidates nominated for re-election, and we welcome direct engagement with our stockholders on any of these matters. If you have already voted and wish to change your vote, see "Can I revoke my proxy or change my vote?" under the General Information about the Annual Meeting section in our Proxy Statement.

We appreciate your continued support.

Sincerely,

Thomas R. Stanton

Chairman of the Board of Directors

ADTRAN Holdings, Inc.

Cautionary Note regarding Forward-Looking Statements

All statements in this additional solicitation material that are not historical are forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995. Forward-looking statements can be identified by words such as "anticipate," "intend," "plan," "goal," "seek," "believe," "project," "estimate," "expect," "strategy," "future," "likely," "may," "should," "will" and similar references to future developments, results or periods. Examples of forward-looking statements include, among others, statements we make regarding our expectations regarding the actions needed to remediate the Company's remaining material weaknesses and the timing of such remediation. Examples of these risks and uncertainties with respect to material weaknesses include, but are not limited to: risks related to our ability to develop and maintain effective internal control over financial reporting and to remediate our outstanding material weaknesses, as well as the risks and uncertainties under the heading "Risk Factors" in our most recent Annual Report on Form 10-K and in our subsequent quarterly reports on Form 10-Q or other filings with the Securities and Exchange Commission. The Company wishes to caution readers not to place undue reliance on any such forward-looking statements, which speak only as of the date made. The forward-looking statements contained in this additional solicitation material represent management's views as of the date of this disclosure. The Company disclaims, however, any intent or obligation to update forward-looking statements, either as a result of future developments, new information or otherwise, except as may be required by law.