That's about an 80 ohm loop for 22-gauge and 125 ohms for 24-gauge wire. This should not be an issue at all.
If the wiring goes outside or underground you should consider adding a grounded gas-tube type protector near the TA900 for safety and to avoid potential damage from induced voltage spikes.
P.S. Beware of the stallion in that closet. He tends to kick.
Dug this tidbit up in my notes from way back....
In most cases the Adtran is within a few feet of the key system, fax, or phone. This means very low resistance in the wiring. Low battery is fine, plenty of loop current over a short loop. In some cases the box may be on a ranch or farm with a long run of wire to a barn half a mile away. Higher battery is needed to ensure enough current flow to reliably signal off-hook and power the phone at the end of the line. Auto-battery senses the actual loop current off-hook and adjusts voltage accordingly.
Hope it helps!