I think you have to use a Special Gateway for the fAX communication. Normal Lync send the call back to the Gateway if the analog device is configured for fax in Lync.
I think you have to use a Gateway with SBC like from Audiocdes MSBG 1000 and route the sip trunk over this Gateway to Lync an connect the anlaog line to this gateway.
regards Holger Technical Specialist UC