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Fax machines connected to a Cisco ATA cannot send faxes, or fax calls fail intermittently
This problem occurs when the fax machine connected to the Cisco Analog Telephone Adaptor (ATA) 188 uses a Super Group 3 (G3) fax protocol. G3 is a standard out of the T.30 and actually can use V.34. It is recognized by voice gateways as a modem call (2100Hz with phase reversals). Super G3 fax machines support speeds of 33.6 Kbps (modem speed), and most of these fax machines use their dual modems to transmit and receive faxes.
The Cisco ATA was designed to support analog phones and G3 fax transmissions (14400 bps maximum). The Cisco ATAs were not designed to support modems either. In short, Super-G3 is not supported by Cisco ATAs.
The Super-G3 fax protocol can sometimes work with the Cisco ATA 188, but there can be issues with this setup just as seen with modems through an ATA.
To resolve this problem, try to modify these parameters:
Change the fax speed to 9600 bps (the recommended speed for no issues).
Set the protcol to G3.
Disable the Error Correcting Mode (ECM).
If the other actions mentioned do not work, plug the fax machine into the Foreign Exchange Station (FXS) port in the router and configure the router for modem pass-through.
Note: If none of these workarounds is successful, a different fax machine must be used.