FXO calls disconnect when answered

Question

Hi
I've been struggling with this issue for over a month and both I and TAC seem to be stumped.
I have a customer with a small office that we are trying to migrate to a centralized CCM cluster. They have a 1760V with 4 analog lines that we want to connect to an FXO card. When we connect the lines to the router for testing I get the following behavior.
PSTN caller calls into the system.
The IP phone user hears the phone ring and answers the call.
The IP phone user sees the call disconnect After a second or two, the call begins to ring again on the IP phone If the IP phone user answers the call again, the call disconnects and the process repeats The PSTN caller hears ringback the whole time.
The problem seems to be that the router is interpreting a disconnect coming from the telco.
Trace snippet:-
008192: .Jul 31 08:36:28.484: htsp_process_event: [2/0,
FXOLS_PROCEEDING,
E_HTSP_CONNECT]fxols_offhook_connect
008193: .Jul 31 08:36:28.488: [2/0] set signal state = 0xC timestamp = 0
008194: .Jul 31 08:36:28.488: htsp_timer_stop
008195: .Jul 31 08:36:28.488: htsp_process_event: [2/0, FXOLS_CONNECT,
E_HTSP_VOICE_CUT_THROUGH]fxols_connect_proc_voice
008196: .Jul 31 08:36:28.716: htsp_process_event: [2/0, FXOLS_CONNECT,
E_DSP_SIG_0100]fxols_normal_battery
008197: .Jul 31 08:36:28.744: htsp_process_event: [2/0, FXOLS_CONNECT,
E_DSP_SIG_1100]fxols_offhook_disc
I've talked to the CO guy but he's been little help. I did have them run a ground start trunk for testing but it exibits the same behavior.
I've tried every combination of battery-reversal supervisory disconnect etc... that I can think of but I cannot seem to solve this problem.
TAC does not think its hardware related but I'm going to send a replacement router out just in case.
I'm hoping someone might have some suggestions for me.
Thanks in advanced.

Answer 1

I have seen this problem before where the polarity on the wires were reversed and/or the physical wiring is physically 4 pairs instead of 2. The physical wires going into the fxo ports should be 2 wire.
Please have the telco verify that the polarity is not reversed [ make sure that verify/re-verify ] as sometimes they claim to check, but are not diligent enough when checking.
If the gateway is somehow detecting battery-reversal, then it will essentially disconnect the PSTN call leg, which will cause the ip phone to see a disconnect. Next, because the PSTN continues to ring the call, a second call will come in [ from the view of the ip phone ]. To test if battery reversal is causing the problem, you can temporarily disable battery-reversal detection on the voice port by using the following command:
router#test voice port 1/1/0 detector battery-reversal
If after disabling battery-reversal, and this problem dissapears, then there is your problem. Please note that the test voice port command only lasts until the router is rebooted.
HTH,
Tony