VEGA Phoenix PSI Data Fax Modem - Manual (Page 107)

AT Commands
692302 6-19
\X Forward XON/XOFF
When the command for software-controlled data flow control (\G1) is activated, the
PC and modem execute data control with XON/XOFF characters. The \X command
specifies whether the control characters are sent from your modem to the partner
modem.
\X0 Detect the XON/XOFF characters without your modem forwarding them
to the partner system (default).
\X1 Detect XON/XOFF characters and immediately forward them to the
partner system.