John,
We finally contacted our reseller and they informed us that the MP-203 is not capable of accepting calls on the FXO port from the SIP side (although there's some documentation on how that could work on the audiocodes web site but we were unable to make that happen).
The FXO port is for outbound only and from the FXS port only (911 calls for example).
At least with the current firmware, so we had to stick to the MP114 or other brands like linksys for the time being...
A shame...