Call collision

In telecommunications, a call collision (commonly known as glare) is one of two things:

1. The contention that occurs when a terminal and data circuit-terminating equipment (DCE) specify the same channel at the same time to transfer a call request and handle an incoming call. When call collision occurs, the DCE proceeds with the call request and cancels the incoming call.
2. The condition that occurs when a trunk or channel is seized at both ends simultaneously.

If you have ever tried to make a call out on a PBX, and been accidentally connected to an incoming call, you have experienced glare. This can sometimes happen at home too, if you pick up your phone to make a call out at the exact second that a call is about to start ringing in.
Multi-line hunting generally avoids glare by picking trunks in opposite preference order so the highest numbered line, which is last choice for incoming calls, is first choice for outgoing calls, like so:

incoming -->1
            2
            3
            4
            5
            6
            7
            8<-- outgoing

For PBX trunks, glare can further be reduced by using ground start trunking. IE: Nortel BSP discourages using loop start trunks for this and other reasons. Long Distance exchanges in the 1950s and 60s incorporated Glare Detectors to alleviate the problem.
 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).