Reference:Configuration/FXS/Physical

The physical settings of the analogue interfaces can be made here:

Pulse:	A checked check box enables the recognition of pulse dialling on the relevant interface.
Reverse:	A checked check box inverts the wiring of the relevant interface. This is only necessary in the event of incompatibility of the terminals, since some terminals (for example, in the US) are wired the opposite way.
First ring followed by CLIP:	This checkbox creates a short ring pulse of 100ms duration followed by a pause of 1 second. This pause is then used to send CLIP information to the phone. Then the ringing switches over to the normal intervals depending on the call being internal or external. If not checked the ringing directly starts with the corrosponding interval. Some telephones can explicitly suppress the first ring just to receive and present CLIP before audible ringing (often called 1.st Ring OFF in menues). If a telephone cannot suppress the first ring or is not configured to do so, this checkbox may be unchecked to immediately start ringing without a short audible ring, which will occur otherwise. Telephones that do not accept CLIP information after the second ring need this checkbox checked to present CLIP correctly. One known model is the T-COM Concept P412.
Receive gain:	Set the loudness a user receives at this interface. Default is -10dB for short lines and -7 db for long lines.
Transmit gain:	Set the loudness a user transmits at this interface. Default is -3dB for short lines and 0 db for long lines. Some fax devices may need a lower transmit level to avoid clipping.
Country:	Set the line impedance for different countries. In Europe CTR21 is used.
Metering pulse frequency:	currently unused
Optimization grade:	As per default the TEL ports are optimized for low power consumption (Optimization 'high' and 'mid'). This decreases the usual on-hook supply voltage of 48V down to 24V. Most conventional phones and faxes can deal with this. In case a special phone is connected that cannot deal with this low voltage this dropdown can be modified. 'high' and 'mid' both set 24V on-hook voltage. The off-hook loop current is set to 20mA. 'low' sets 48V on-hook-voltage. The off-hook loop current is now configurable in three steps :
Loop Current:	As described before this topic changes the off-hook loop current and can be modified if 'Optimization grade' is set to 'low'. 'low' sets 20mA, 'mid' set 23mA, 'high' sets 26mA.
RingTrip Threshold:	This topic modifies the current-threshold which lets the gateway distinguish between a ringing condition and a hook-off condition while the ringing voltage is applied from the gateway to the phone. Crossing this threshold by the phone, the gateway assumes a hook-off and therefore stops applying the ringing voltage and enters normal off-hook state. Imagine you encounter a problem that looks like this : very short time ringing, somehow immediate off-hook and then on-hook transition again - this call is lost. This looks like your phone requires too much current for ringing that makes the gateway wrongly assume a hook-off. In this case it is recommended to try the higher threshold currents. 'low' sets 34mA, 'mid' sets 37,5mA and 'high' sets 40mA.
Ringing Voltage	This topic modifies the ringing voltage used for ringing. 'low' sets 65V, 'mid' sets 68V and 'high' sets 70V AC.

Pinout of the a/b connector

Pin	Function
3	TIP
4	RING