Low volume of called person's voice with Asterisk+Avaya PBX

I have an Asterisk system (current version of Asterisk@Home) fitted with a Digium TDM11B - 1 FXO + 1 FXS. A standard phone is plugged into the FXS port and the FXO port is connected to an analogue port on an Avaya Ipoffice 412 PBX.

Calls made from the phone into Asterisk services - fax, recorded messages, etc, all work fine. Calls made from the phone routed to the outside world through the Avaya PBX connect OK, and the volume heard by the receiving party is fine, but the volume from the receiving party (what you hear when they speak) is very low.

I have tried a number of different combinations of values for rxgain and txgain in /etc/asterisk/zapata.conf, but the received volume is still unacceptably low.

Is there any way in zapata.conf of specifying different rxgain + txgain values on a per-channel basis?

Do I need a special cable to connect from the Asterisk FXO port to the Avaya PBX analogue port? At the moment I am using the same kind of connection that we use to connect telephones or modems to PBX analogue ports. I have tried different types of BT adaptors - PABX Master, PABX Secondary, etc - but this does not seem to make any difference.