Not all phones in queue or ring group ringing


We have a Freepbx version running on 6.12.65-24 and this started to see random issues with some phones in a queue or ring group not ringing. There sems to be no pattern to which phones will ring in either and they are connected to different switches. We have 3 Cisco 3524 XL switches, 2 connected together using gigastack and the other is in a different comms cabinet on the other side of the building. Members of the queue in question are connected to the two switches whereas the members of the RG are on another. There are no wrap up delays on the queue either. We did change the dialplan.xml file last week to reduce the dial time to zero, from 5 seconds, but this has now been reverted back.

Any thoughts of where to look next?