Are you testing the voltage at the port on the adtran or at the end of the cable pair and what voltage are you reading? If you already are testing directly at the adtran, is this low voltage consistent across all FXS ports on the device?
If it's an issue getting the intercom system to answer the call, then the ring voltage can be adjusted on the physical FXO port page of the adtran. We've encountered issues where paging systems expect 70~90v minimum to answer, and although the ring voltage tops out at 70v that fixed it for us.
The on-hook voltage is indeed -48 volts. If you're measuring it with the unit off-hook it will be substantially less. This is normal on all analog telephone circuits.