You either need SIP extensions on OXE side and you should check/test MWI compatibilities for hotel mode for your software release, or use FXO to SIP gateway on OXE side (connect analog users to FXO gw on OXE side) and link it to terminated FXS ports.
Something like this:
1. OXE SIP -->ONT netw (SIp transport) -->SIP to FXS GW (VRGIII-31412-N)--> phone
2. OXE analog pots --> FXO to SIP gw (whatever vendor) --> ONT netw --> SIP to FXS GW (VRGIII-31412-N) --> phone
For first scenario you must have OXE software license over R9 and SIP user licenses.
Look also here:
viewtopic.php?f=230&t=4493&hilit=hotel+sip