When set to ENABLE, on reception of a "183 Session Progress" SIP message,
the gateway will stop playing the ring tone and will let the audio coming
from the SIP side flow to PSTN side.
When set to DISABLE, the audio coming from the SIP side won't be relayed
to the PSTN side until the gateway receives a "200 OK" SIP message.
NOTE: When the FXO device hooked to the FXS port is a FAX, this
parameter MUST be set to ENABLE so that the gateway can
automatically switch to FAX reception mode when it detects a FAX tone.
Otherwise, you might have some problems with FAX transmissions.