timing-source t1 0/4
Just to be clear the above command is added to the second and every other slaved switch. It is not added to the Primary switch.
Telco normally want to be the timing source for PRIs.
Absolutely agreed. In this case, telco isn't involved on any TDM circuits. OP has SIP from telco. The only T-1s are local, one from each of two TA900s to the PBX. Customer wants redundancy should either of the TA900s fail. Normally the TA900 would source clock and PBX would slave to it. In order to achieve redundancy, we can generate clock from the PBX and sync both TA900s to the PBX clock. Now, should either TA900 fail completely, the other will stay in sync with the PBX.