You can use com0com drivers + t38modem along with Windows Fax & Scan to put in place a -free- Fax over IP solution.
The overall design would be :
WIndows Fax & Scan <=com0com=> t38modem <=SIP/H323=> Voice Gateway <=> PSTN
OR
WIndows Fax & Scan <=com0com=> t38modem <=SIP/H323=> CUCM <=whatever-yoy-want=> PSTN
Find com0com and t38modem Windows Binaries here