How do I set the Adtran to be the clock source for the PBX for a Dual PRI scenario?
Is the only thing I need?
If the TA908e will be providing the clock for both PRIs such as feeding a PBX, yes. Make sure that the PBX is set to take its clock from the line.
If there's a T1 coming from a carrier into the TA908e, you probably want that T1 interface to be the timing-source for the network. The TA908e will sync to that clock and pass timing through to the remaining T1s.
It can get interesting in some complex scenarios such as where the TA908e and a separate unrelated carrier T1 are both feeding the PBX.
When finished, type "show interface | i Slip" two times about a minute apart and make sure that the Controlled Slip Seconds aren't increasing on any operational T1s to verify.