Fax Voip Windows Fax Service Provider (Fax Voip FSP) enables Microsoft Fax and Windows Fax and Scan, which are built right into Windows, to send and receive T.38 and audio (over G.711 codec) faxes using your VoIP (SIP or H.323) infrastructure without any further cost. Fax Voip FSP does not use Virtual COM Ports (modems). This allows you to use ECM (Error Correction Mode) and greatly improve reliability of your fax system.