Reference10:Interfaces/FXO/Signaling

The call signalling settings of the analogue FXO interfaces can be made here:

Disable:	Disables the relevant analogue FXO interface.
Speech Bearer Capability:	Calls on the relevant interface are transmitted with Audio Bearer Capability as standard. A checked check box transmits calls from the relevant interface with Speech Bearer Capability. This only makes sense if only telephones are operated on the relevant interface (no fax machine or modem).
Central Office Tone detection:	When the FXO initiates a call, normally a audible tone is expected to be received from the central office/local PABX side. The tone detection works independantly from any country setting. When the expected tone cannot be detected within 2.6sec, then the call will be cancelled. If this tone detection is disabled, the FXO will continue dialing DTMF tones of the configured number, with a fixed delay of 800ms after hook-off.
Alert Tone detection:	After the configured number has been dialed, the FXO normally waits for a audible response from the dialed side that can be identified as an alerting tone. The proper detection and analysis of this tone takes up to a few seconds. If you additionally enable the Assume alert option, the detection is simplified and therefore reduced to a simple tone detection. This means that an alert will be regarded as detected no matter what tone or rhythm is provided. This is much faster than the exact analysis and normally has no negative effect. When an alert is detected, the voip>-FXO->peer connection is switched through (connected), so that the voip initiator of the call now can hear the alerting tone from peer side. Disabling Alert Tone detection will directly connect the call after dialing is complete, which is much faster.
Assume Alert:	see previous topic for details. Assume Alert has no effect when Alert Tone detection is disabled.
Establish call on line polarity reversal:	When the FXO port detects a reversed polarity and the FXO port is idle, a new call wil be established. Countries that use DTMF CallerId also often use a Polarity Reversal preceding the DTMF CLIP sequence and start with AC ringing afterwards. In detail the sequence is as follows : Line polarity reversed, 200ms pause, DTMF CallerId transmission, Line polarity normal, 1000ms pause, AC ringing, pause ... . For instance Sweden is one of those countries that use DTMF CallerId combined with Line polarity reversal call establishment. If this checkmark is unchecked here, the call will miss the DTMF CallerId sequence, when established with the AC ringing. Note that this option can also be used with FSK CallerId.
PSTN drives conversation state in reversed line polaritity:	When the FXO port detects a reversed line polarity, the connection will be regarded as connected. If the line polarity is switched back to normal polarity, the current connection will be terminated. This behaviour is common for PSTN in Sweden.
Caller ID 1 standard:	Selects the standard in which CallerId/CLIP is detected and decoded. CallerId carries the caller's phone number in case of an incoming/ringing call. This number is fed into the resulting voip call as the CGPN (Calling Party Number). Note that for this reason incoming calls from the FXO port must use en-bloc delay dialling to allow for receipt of the CLI. This may be done by registering the Port with a Gateway type PBX object and set the Enblock Count option in this object or use a route with Force enblock set in the Gateway->Routes Menu. FSK CallerId standards can contain additional information like Called Line Id or Date/Time information. These additional informations will be dropped. DTMF CallerId only contains the 'Calling Line Id'.