In telecommunication, the term call collision (glare -- a deprecated synonym) has the following meanings:

• 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.
• The condition that occurs when a trunk or channel is seized at both ends simultaneously, thereby blocking a call.

Source: from Federal Standard 1037C and from MIL-STD-188