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 CB24 FXO Ports Do Not Hangup, or Are Slow to Hangup
Solution Having trouble with FXO ports hanging after a call is ended?  Most likely the issue is analog related since the ports all work, but have a delay.

The analog world operates around changing or the total removal of power, called "battery" from the line to signal the state of the call. The two wires, Tip and Ring, can have a DC voltage, a AC voltage, or no voltage at all to signify what is "happening" on the line. Here is a brief summary of what can happen there.

Idle - nominal 48VDC on the line, phone is on-hook and nothing is happening
AC Voltage - ringing, normally above 60VAC RMS
Off-Hook - 5VDC-12VDC as defined by the load (telephone or FXO port)
Zero Volts - line inoperable, or far end disconnect if the caller has hung up

The normal process for an inbound call is for the line to go into the follow state change:

Idle - nothing happening
Ringing - caller has dialed this specific number
Off-Hook - called party answers line

At this time there are two branched to the call -- the far end caller can hang up, or the called party hangs up. If the called party hangs up, the local PBX knows this and simply lets go of the phone line (basically hangs up the phone).

This is where your problem most likely comes from -- the next scenario is that the far end caller hangs up. The phone company tells the PBX this by pulling battery (zero volts) on the analog line, which is detected by the PBX as the far end has hung up. This pause in battery can be 100ms to over a few seconds. This event is a standard and if you think about it, there is no other method to tell the local PBX that the far end has hung up other than this method, since the interconnection is analog.

So, your problem could be one of two items -- the FXO port is not detecting this event, or the event never happens. The fact that the call holds for more than a few minutes tells me that the event has not happened, and what IS detected is a recycling of the phone line power at a later time by the phone company since they did not see the line go to the on-hook status by measuring the change from the lower DC voltage to the higher DC nominal voltage seen in the IDLE state. This is the time when you do not hang up the phone, and you start to get a loud beeping and a recording telling you to hang up, eventually that goes to a battery pull event too until the line is taken back to idle at the PBX.

How can you see this happening?

The CB24 has a wonderful debugging aid, the LCD front panel. You can "see" every state of the analog line there on the display.

In LOOP mode, here is the state table as seen from the TX line.

B - Idle
0 - ringing
B - off hook (this is strange I know since Idle is also B, but the RX side should be a 1 instead of a B when in this state)
L - zero volts on the line

In theory, you can "see" the disconnect when the far end caller hangs up of 100ms to a few seconds at the end of the call by watching the display blink to a "L" for that brief period when battery is pulled. You can also see this more statically by disconnecting the line from the CB24 port and essentially pulling battery from the line, when it will stay in the L condition until you put the line back in place.

Keywords

disconnect supervision CPC hangout FXO



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