ADTRAN Holdings Inc.: Bekanntmachung der Einberufung zur Hauptversammlung am 13.05.2026 in www.virtualshareholdermeeting.com/ADTN2026 mit dem Ziel der europaweiten Verbreitung gemäß §121 AktG

NOTICE HEREBY IS GIVEN that the 2026 Annual Meeting of Stockholders (the “Annual Meeting”) of ADTRAN Holdings, Inc. (the “Company”) will be held on Wednesday, May 13, 2026, at 10:30 a.m., Central Time, via live webcast. You can participate in the meeting by visiting www.virtualshareholdermeeting.com/ADTN2026 and entering the 16-digit control number included on your Notice or proxy card. If you hold your shares through an intermediary, such as a bank or broker, and do not have a control number, please contact the bank or broker. Please log in to the website by 10:15 a.m., Central Time, on the day of the meeting. There is no physical location for the Annual Meeting.

The Annual Meeting is being held for the purposes of considering and voting upon:

Stockholders will also consider and act upon such other business as may properly come before the Annual Meeting or any adjournments thereof. The Board of Directors is not aware of any other business to be presented for a vote of the stockholders at the Annual Meeting. See page 90 of the Proxy Statement for additional details.

How to Vote

The Board of Directors recommends that you vote FOR each of the six director nominees; FOR the approval of the amendment to the Company's Amended and Restated Certificate of Incorporation, FOR the approval on an advisory basis of the compensation of the Company’s NEOs; and FOR the ratification of the appointment of PricewaterhouseCoopers LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2026. The Annual Meeting may be adjourned from time to time without notice other than announcement at the meeting or at adjournments thereof, and any business for which notice is hereby given may be transacted at any such adjournment.

Information relating to the above matters is set forth in the Proxy Statement. Stockholders of record at the close of business on March 16, 2026, are entitled to receive notice of and to vote during the Annual Meeting and any adjournments thereof.

Whether or not you plan to participate in the Annual Meeting, we urge you to review these materials carefully, which are available at https://materials.proxyvote.com/00486H. We also encourage you to vote by (i) following the instructions on the notice that you received from your broker, bank or other nominee if your shares are held beneficially in “street name” or (ii) one of the following means if your shares are registered directly in your name with the Company’s transfer agent:

By Internet: Go to the website www.proxyvote.com and follow the instructions. You will need the control number included on your Notice to obtain your records and vote by Internet.

By Telephone: From a touch-tone telephone, dial 1-800-690-6903 (toll-free within the United States; calls from Germany or other countries may incur standard international charges) and follow the recorded instructions. You will need the control number included on your Notice in order to vote by telephone.

By Mail: You may request a hard copy of the proxy materials, including a proxy card, by following the instructions on your Notice. If you request and receive a proxy card, please mark your selections on the proxy card, date and sign your name exactly as it appears on the proxy card and mail the proxy card in the pre-paid envelope that will be provided to you. Mailed proxy cards must be received no later than May 12, 2026 in order to be counted for the Annual Meeting unless you are voting shares of common stock held in our 401(k) plan, in which case the deadline is May 10, 2026.

You can submit questions in advance of the Annual Meeting by visiting www.proxyvote.com, entering your 16-digit control number, and using the “Questions for Management” feature. You can submit questions during the Annual Meeting by following the instructions on the meeting website. Management will answer pertinent questions in the “ Investor Resources” section of our website at https://investors.adtran.com. The questions and answers will be available as soon as practicable after the Annual Meeting and will remain available for thirty (30) days after posting.

For organizational questions regarding the Annual Meeting of Stockholders, stockholders holding their shares in Germany may also contact our German hotline, operated by ADEUS Aktienregister-Service-GmbH, Königinstr. 28, 80802 München,

Huntsville, Alabama, United States of America, March 27, 2026