- A subsidiary of Mauritius telecom
- A Telecom
- AAPT
- Abakan
- Abatel
- ABN-AMRO
- Absec
- ACC Telekommunikation GmbH (ACC Telecom)
- Access World
- Accos
- Ackermann
- ACS
- ACS Telcom, Inc
- ADDX
- Advanced Business Services
- Advantage Communications Management, Inc.
- Aero LLoyd
- Afghan Wireless
- Afric Phone
- Africa
- Africa 2000
- Africa Access
- Africa News
- Africa Phone
- Africa Titoko
- Africa Way
- Afridylle
- Afrik
- Afro 2000
- Afro Touch 4 You
- AGT
- Aircom
- Airtel
- AIS
- Ajax mobile
- Al-Assadi
- AL-NET
- Albert Heijn
- Albtelekom
- Alcatel
- Alfa 902 Telecom
- Alfatel Telecommunications
- Alhermij
- Allianz Arena
- Allo
- Allo Asie
- Allo Chine
- Allworld
- Alpha Telecom GmbH
- Altus
- Amazônia Celular
- AMC Amsterdam
- AmCall/AmCall (IEM)
- Amena
- America First
- American Express
- American Express Telecom Inc.
- American Telecom
- American Telephone & Telegraph Company
- Ameritech
- Ameritech Communications International, Inc.
- AmeriVox
- AmeriVox Benelux
- AmeriVox Transworld
- Amimex
- Anaconda
- Andusa
- Anritsu
- Antel
- Antel Uruguay
- Antel, Inc.
- ANWB Travelcom
- Any
- ANY phone
- Apelcom
- APG
- Aplab
- Apollo
- Arabel
- Arabtel
- Aras
- ARC
- ARC Networks
- Arena AufSchalke
- Aria
- Arora Telekommunikation GmbH (ATG)
- AS Communications
- Asia Love
- Asia Plus
- Asia Telecom SL
- Association des Télécommunications
- AT&T Alascom
- AT&T American Telephone & Telegraph Company
- AT&T Wireless
- ATI
- ATL
- Atlantic Telecom
- Atlantic Telecom France
- Atlas Telecom
- ATX Telecomminications Service
- Auna
- Australia Post
- Authentic Telecommunications, Inc.
- Avant
- Avantel
- AVE
- Avea
- AVM
- Axalto
- Axis Technologies Solutions (M) Sdn. Bhd.
- AXS Telecom
- AXXON Telecom GmbH
- Ay Cell
- Ay Yildiz
- Azul
- Azultel