We have good experiences with the XCAPI Software from TE Systems for GFI Faxmaker integrations:
http://www.xcapi.de/prinfo/index.en.php
EICON also distributes this product as an OEM Version, but the EICON DIVA SoftIP is crap because it is instable and outdated from the feature support.
It simulates basically a virtual Fax card and can connect to the CallManager or the Gateway through H.323 or SIP.
Regards
Thomas