did you verify if the calls work
fine from the ISDN to a phone connected to the PBX too?
This is to verify that you dont have
a clocking issue on the 2Mbps ISDN trunk..
I believe that the issue is only encountered at the remote site –
i.e. through a call place over the SIP trunk, but I will need to check on this.
This means that normal calls to the main office all work correctly. It is only
when the call is transferred to a remote location that the issue arises. I also
believe that the issue can be reproduced simple by making a call to a remote
extension from the main office which would just use obviously the SIP trunk (no
ISDN/PSTN involved).