Queue Ring Back Tone Incorrect?

So we’ve “gone live” with our new asterisk phone system in our office, and its working quite well.

Working thru various niggles and i’ve hit one which has me stumped.

We have two incoming trunks, one on SIP from a commercial VOIP service, and a BT ISDN 30 service feeding into a Sangoma A101DE.

Multiple DDI’s on each service are routed into various time conditions, and then on to a Queue with some extensions inside.

This all works great, calls come in, are correctly routed and tagged and come thru to the operators just as they should.

However, we’ve noticed an issue with the ring back tone produced (the queues are set to “Ringing instead of MOH”) when a caller dials in.

If the phone number used happens to be a SIP number, everything sounds right, callers are presented with a standard UK ringing sound. If however you dial a number which comes in over the ISDN trunks, the ringing sound is not a standard UK sound, instead its a single long ring.

This only seems to happen when directing an ISDN call to a Queue. If the ISDN call is directed to a ring group, or directly to an extension it rings normally…

Any ideas on whats causing this?