I actually got a call back from Adtran today and they told me if the 624 was an L3 version, which it is, then the FXO ports can be configured for analog DID service to a PBX. I guess I'm going to find out if this is true or not.

Majestic, old school analog DID's are just designed that way, they don't give you dial tone when you go off-hook with a buttset, as they are for inbound traffic only. Two-way DID didn't come about until supertrunk T-1's came out(at least that's what we called them on the west coast)and then the next step was PRI.