Symptom
A PSTN user calls an intra-office user through the FXO port on the IAD. When the intra-office user is in busy state, the PSTN user hears two-second ringback tone before hearing the busy tone.
Possible Cause
The possible cause is the technical restriction.
Two carriers are involved in the service through the FXO port on the IAD. The two-second ringback tone is played by a network and cannot be canceled another network. In addition, it takes 2 seconds for the IAD to send a detection message to the SoftCo, which is normal for message exchange. Therefore, it is inevitable that the calling party hears two-second ringback tone before hearing the busy tone.
Solution
Explain the technical restriction cause to the customer. Check whether the customer can accept the cause.
- If yes, the procedure ends.
- If no, replace the AT0 trunk with a more expensive trunk like PRI to connect to the PSTN network.
