VoIP Speak writes about Fax for Asterisk:
Fax For Asterisk inter-operates with standards-compliant fax machines connected to Asterisk 1.4 and 1.6 on x86 Linux systems. It provides low-speed PSTN faxing via DAHDI-compatible telephony interface cards as well as VoIP faxing to T.38-compatible SIP end points and service providers. Fax For Asterisk operates at speeds up to 14.4kbps and supports V.17, V.27 and V.29 fax modems. .... Fax For Asterisk is available free of charge from the Digium webstore for one concurrent fax session. Multi-session licenses are available for a one-time fee of $38.50 per channel. Fax For Asterisk is available immediately. Fax capabilities for Digium’s Switchvox IP PBX were announced in February of this year and are based on this solution.