When calling inbound from the PSTN into any CUCM registered IP Phone, the ISDN cause code of 80B1 – quality of service unavailable was displaying in the debug isdn q931 output.

The CUCM shows not activity coming through from the H323 Gateway at the Branch.

I found the Router had been Call Fallback Active configured with some prob config on the dial-peers going to CUCM Servers.

Once I removed the Call Fallback active cmd, the voice calls were successful.

HOSTNAME(Config)# no call fallback active