Configure Fax T38 C

To enable T.38, follow these steps:

1. Make sure "Enable Fax/Modem Relay" is enabled.
2. Make sure "Detection Mode" is Standard.
3. Make sure "Relay Mode" is T.38.
4. Make sure the Use V.34 is unchecked. (For reliability reason, some faxes failed to failback to V17)