| 56 kbps | 56k line | 5ESS Switch | 610 | A&B |
| AM | ATM | Adaptive Communication Environment | Adaptive Digital Pulse Code Modulation | America On-Line, Inc. |
| Amplitude Modulation | Asynchronous Communications Interface Adapter | Asynchronous Transfer Mode | Automatic Number Identification | Automatic Repeat Request |
| Basic Rate Interface | Baudot code | Bit Error Rate | Bose-Chaudhuri-Hocquenghem Code | CHEOPS |
| CTI | CTS | CU-SeeMe | Call Unix | Caller ID |
| Carrierless Amplitude/Phase Modulation | Cellular Digital Packet Data | Central office exchange service | Clover | Code Division Multiple Access |
| Committed Data Rate | Committed Information Rate | Common Intermediate Format | Computer Telephone Integration | Conferencing over IP |
| Customer Information Control System | DPSK | DS level | DS0 | DS1 |
| DS1C | DS2 | DS3 | DSL | DSU |
| Data Communication Equipment | Data Over Cable Service Interface Specification | Data Terminal Equipment | Data Terminal Ready | Dataphone Digital Service |
| Delphi | Dialled Number Identification Service | Digital Data Service | Digital Enhanced Cordless Telecommunications | Digital Radio Mondiale |
| Digital Simultaneous Voice and Data | Digital Subscriber Line | Digital Switched Network | Direct Inward Dialing | Dual Tone Multi Frequency |
| E-carrier system | E1 | E2 | E3 | E4 |
| E5 | EIA-232 | EIA-232C | EIA-422 | EIA-423 |
| EIA-449 | EIA-485 | EIA-530 | Eggdrop | FM |
| FPLMTS | FRAD | Fax over IP | Fibre Channel | Foreign eXchange Office |