T.38 is the only facsimile standard designed to implement real-time fax transportation over Internet (also known as FoIP, fax over IP). In contrast to G.711 "pass through" faxing where the fax call is carried in a VoIP call encoded as audio, T.38 is designed especially for fax data transfer and includes procedures focused on data loss elimination.
VentaFax & Voice software supports T.38 standard for sending end receiving faxes across IP networks. You may download and install VentaFax in order to try sending faxes via T.38 before you buy it.
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Read about T.38 standard in Wikipedia