Multitech MMH900 User Manual page 174

Appendix J - Internal MMH2834 Composite Link Modem Commands
Five different X commands are available (X0 through X4), with
five different effects on the MMH2834's Result Codes.
X0 Provides the basic (short) result codes and provides
"dumb" dial capabilities.
X1 Provides the extended result codes and provides "dumb"
dial capabilities.
The remaining X commands affect Call Progress, and turn on
Extended Result Codes.
X2 Looks for dial tone only and will not provide a busy
response.
X3 Looks for busy only and not look for dial tone.
X4 Looks for dial tone and for busy.
The factory default setting is X0, which selects Basic Result
Codes and the blind ("dumb") method of dialing.
174