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Busy Tone Is Heard Immediately After the Ringback Tone Is Played Once in an Outgoing Call Through the FXO Port

Publication Date:  2015-07-24 Views:  361 Downloads:  0
Issue Description
Network: IAD1224-U1980-IAD132-Foreign exchange office (FXO) port-Public switched telephone network (PSTN)

Version: IAD132 eSpace IAD V300R001C07SPC200

An IAD1224 user can make intra-office calls properly. When making an outgoing call through the FXO port on the IAD132, the user hears the busy tone immediately after the ringback tone is played once.
Handling Process
Step 1 Connect downlink PSTN subscriber cables directly to the POTS. Calls can be made properly, which indicates that the the cable connected to the FXO port is not faulty.

Step 2 Connect one pair of PSTN subscriber cables to a phone and another pair of PSTN subscriber cables to the FXO port on the IAD132. IAD1224 users can make outgoing calls through the FXO port properly.

Based on the analysis result, the possible cause is that the interval between simulated off-hook and dialing is too short for the carrier side to collect the number.

Step 3 Run the sip soft-parameter send-dtmf-delay-time<350-65535> command on the IAD132 to set the dual tone multiple frequency (DTMF) delay. Increase the send-dtmf-delay-time value from the default value 350 for debugging. When the value is set to 900, the fault is rectified.

Root Cause
1.  The cable connected to the FXO port is faulty.

2.  The called number is not properly sent.
When a user makes an outgoing call through the FXO port on the IAD, the call is first routed to the FXO port. Then, the FXO port performs simulated off-hook and sends the number. By default, the send-dtmf-delay-time value is set to 350, in milliseconds. That is, the FXO port sends the number when 350 ms has elapsed after simulated off-hook. If the peer end detects FXO off-hook but has not switched the state to digit collection when the FXO port sends the number, the number cannot be sent completely, and the call fails.