I am running a Xorcom Asterisk PBX with Elastix 1.6 (Asterisk 1.4.31). When calling internal extensions (all are Cisco phones of different models with firmware ver 8-5-4TH1-6) after 7 rings the call goes to a busy tone even though it shouldn't and for no apparent reason. This only occurs from the Cisco phones themselves. Via sip software this doesn't occur. Also we have an IVR and any external call to the extensions through this also work fine either via landline or mobile.
The support tech from Xorcom says it's the calling phone causing the disconnection.
Any ideas what this could be.
I've attached a sample config but I don't think anything here is at fault.
Any help appreciated,