One way audio is caused by NAT/firewa//STUN misconfiguration in most cases. Bot PAP2T and your xDSL device needs to be configured in compatible way.
If you can hear no remote side, then icomming RTP stream either doesn't pass trough xDSL into internal network at all, or it such stream is translated an unexpected way so it's missed by PAP2T.
Catch the network communication between xDSL and PAP2T. If you will see no incomming RTP stream the xDSL configuration needs to be modified. If you will see a RTP stream, it's parameter together with catched SIP dialog packets may help you to narrow the issue ...
Network Address Translation (NAT) Settings for Lines on PAP2T and SPA3102 Phone Adapters
The objective of this document is to configure Network Address Translation (NAT) settings for the lines of communication on the PAP2T ...
Speed Dial Settings on SPA100 Series
Speed Dial Buttons are often used to make the process of calling regularly used contacts quicker and with one button. This option is used in place of dialing the entire phone number in. ...
Firmware Upgrade on SPA112 and SPA122
To upgrade the firmware a specific file must be downloaded from the Internet and uploaded to the device. Upgrading your devices can be very useful. Firmware upgrades can fix software bu...