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Why do incoming calls disconnect after about two rings?


Using Atlas 550 in a "rabbit hole" configuration for a central station alarm receiver. If the receiver (or whatever is answering incoming calls) does not pick up quickly the call will disconnect. I believe this may be the Atlas, is there a setting we can change to increase the amount of time it rings?

Also, not sure if it is related; we are getting many "DS0 offhook - originate timer expired" messages in the event log.

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Re: Why do incoming calls disconnect after about two rings?

Thank you for using the ADTRAN Support Community Forums.

The call is disconnected because of an ISDN Timer, and the timer expires because of the mechanics when doing the "rabbit hole." The RBS side of the rabbit hole is doing Feature Group D signaling, which is basically E&M Wink where the DID digits transmitted after the "wink" are sent in the format of *ANI*DNIS*. The problem is that the "wink" can't be sent until AFTER the FXS line is answered (because that is when the DID digits need to be transmitted - not before the call is answered). The *ANI*DNIS* digits can't be transmitted until after the receiver answers the phone, or they won't be heard by the receiver, and they digits aren't sent until after the "wink" so the "wink" can't be sent until after the FXS line is answered.

On the PRI side, the SETUP message is sent to the ATLAS, and the ATLAS sends a "proceeding" message as an acknowledgment. This starts a 10 second timer that will terminate the call if there are no other update messages. The problem is, that there is no other messages until the WINK is received on the RBS T1 - which doesn't happen until after the call is answered. So with the standard North American ring cadence of 2 on and 4 off, you end up with about 2 rings before that 10 second timer expires and the call is terminated by the PRI switch.

E&M Wink, and Feature Group D were designed to connect to a PBX, where the PBX immediately sends the "wink" when notified of the incoming call. When the Telephone Company switch receives the "wink" it will start to play ringback to the caling party, and the PBX receives the DID digits and routes the call to the appropriate extension; sending ring voltage to that phone. The call is not connected until the extension is answered.

So since there is no PBX involved in the "rabbit hole" configuration and it is all handled by the ATLAS, you have the 10 second (or 2 ring) timeout on the PRI circuit. There is no way around this without introducing a PBX, or some other device that will send a wink, receive the digits, and then route the call before the call is answered.

The "DS0 offhook - originate timer expired" message means that one of the FXS ports is staying offhook longer than the END OF NUMBER TIMEOUT, which can occur if the call is disconnected and the receiver does not hand up the FXS port for several seconds. If there is simply a delay in the receiver hanging up then it isn't anything to worry about, but if the FXS ports are staying in an OFFHOOK condition then you need to look into it further. You look at the FXS status under the MODULE and verify the ports go to an IDLE state between calls. If any stay in an OFFHOOK state there is a problem.

Hope this helps,