The 4x0-DS trunk card supports up to four incoming analog CO (telephone company) lines. The 4x0-CI trunk card supports analog CO lines with Caller-ID. The 4x0-DID trunk card supports up to four inbound-only analog DID trunks. Each DID (direct-inward-dial) trunk can carry up to five virtual phone numbers, which the system will then route to a particular extension. 4x0-DID cards must be housed in an M12x0 trunk module to work properly. 4x0 trunk cards can be added to a CICS system (maximum of two cards, but no 4x0-DID) or an MICS system, which can hold two in the base cabinet, and more in M12x0 trunk modules for expandability. The cards are easy to insert and only require a flathead screwdriver to pry open the clips.