Issues with making calls

We have had a couple of issues with our primary PBX (it is in a hot spare configuration). We have experienced twice now new calls not being able to be made or received, this affects internal and external calls.

Strangely existing calls are not dropped.

Rebooting the primary PBX fixes the issue.

Does anyone know what may cause this issue?

Insufficient information.

Apologies, can you confirm the additional information you need to help with this issue.

Description of how the system is organised, in particular the channel technology used, and if VoIP details of firewalls, NAT, etc.

Protocol trace for the channel technology being used.

The relevant part of the dialplan.

Verbose level 5 trace of the execution of the dialplan (debug levels may prove necessary as well).