As a FoIP (that is "Fax over IP") program, VentaFax supports both T.38 real-time fax standard and G.711 "pass through" faxing where the fax call is carried in a VoIP (that is "Voice over IP") call encoded as audio. It prefers T.38 FoIP standard if supported by Internet telephony server, otherwise G.711 voice codec is used to transfer a fax over IP.
You may download and install VentaFax & Voice in order to try faxing over IP before you buy it.
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Read more about VentaFax & Voice software
Read more about using VentaFax & Voice as a FoIP client
Read about T.38 standard in Wikipedia
Read about G.711 voice codec that can be used for FoIP in Wikipeadia