Phones often get "unknown" call from PSTN trunk, If you answer the call, they can’t hear anything.
It comes from 3 different situation, Ring tone, Polarity and MWI.
For Ring tone and Polarity both of them caused by the PSTN trunk stability which the lines would generate either ringing jitter or polarity signal.
Regarding to MWI, it is depends on the PSTN provider, most Mid-east country their PSTN provider enabled this function, once PBX get MWI it will trigger an incoming call. For U-Series & TA FXO gateway, you could refer this guide: Unknown Calls Issue after Call Hang-up from PSTN line. For S-Series, the MWI issue has been avoided by default.
When you replicate the issue, please post us the following info:
- Asterisk CLI logs. For S-Series: How to Capture Asterisk CLI Logs for Yeasatr S-Series VoIP PBX. For U-Series & Gateway: How to Capture Asterisk CLI logs for Yeastar Devices
- Analog port signal capture: How to Get the FXO/FXS/GSM Port Signal Capture