We have a PABX hanging off an H.323 gateway. If an inbound call from the PSTN presents digits will not match in the CallManager then that call gets offered over E&M ports to the PABX.
Today this happened but the PABX did not match on those digits either and so it sent the call back out to the router which sent it out the PSTN.
Eventually, we had 66 call legs and all ISDN channels in use and about 17,000 call attempts with 3500 actually completed on Call Manager Serviceability Real-Time-Monitor.
Is there any way to prevent this type of phone "loop"?