We have done some thing similiar,
We have extracted the clock from the E1 on which C7 signalling comes from the PSTN.like E1 1 extracted clock from LEVEL 1 tax and E1 2 Extracts clock from International GateWay and the 3rd clock is from our own Stratum 1.
We have used the Logic of SINK CLOCK.. that is the average of all the clock is taken..