Can I FAX reliably through a VoIP channel?

Not usually, and this has nothing to do with whether you are using spandsp or some other FAX modem/machine. If you use a low bit rate codec, such as G.729, you will not be able to get any fax functionality at all. If you use an A-law or u-law codec you will get your fax through most of the time. However, it is unlikely to be totally reliable, for a number of reasons. First, VoIP connections are usually subject to packet loss. This will kill a fax transfer. If the packet loss rate is low a resend may get the page through successfully. If you are on an in-house LAN, packet loss might not be an issue. However, VoIP transmitters and receivers are not usually synchronised. Sometimes the receiver has to insert extra audio, or drop some, to adjust the receive timing. If this happens, a fax transfer will fail.