| Abbreviation | Definition |
|---|---|
| CI | CHARM Interface |
| CI | Customer Installation |
| CIC | Carrier Identification Code |
| CICT | Commission on Information and Communications Technology |
| CID | Call Instance Data |
| CID | Cell IDentification number |
| CID | Craft Interface Device |
| CIL | Call Identification Line |
| CIP | Carrier Identification Parameter |
| CIP | Channel Interface Processor |
| CIPoA | Classical Internet Protocol over ATM (Asynchronous Transfer Mode) |
| CIPT | Changchun Institute of Post and Telecommunications |
| CIU | Communications Interface Unit |
| CKT | Congo Korea Telecom |
| CLASS | Cell Level ATM (Asynchronous Transfer Mode) Services Simulator |
| CLASS | Custom Local Area Signaling Services |
| CLD | Cross-Layer Design |
| CLECS | Competitive Local Exchange Carriers |
| CLETS | California Law Enforcement Telecommunication System |
| CLI | Calling Line Identification |