Adtran n00b here. Will be using an Adtran to hand off from a SIP trunk to POTS on a PBX. From my experience with other hardware manufacturers, this needs to be done with an FXO port, but from what i can tell, the Adtran TA 900's cant do more than a couple FXO lines (I think I saw the 924 can do up to 8 FXOs). If i need to hand off, say 16 POTS to a PBX, what Adtran device can I use to get 16 FXO ports on? or will FXS ports work?
FXS is what you want. It acts like a phone line. FXO is what is on the PBX for CO lines. It acts like a phone. You should be fine.
FXS on Adtran:
Expects loop closure (or ground-start)
FXO on PBX:
Provides loop closure (or ground-start)
When dealing with analog lines, it's important that the Adtran and the PBX be connected to earth ground, preferably at the same point.