When making calls out the PSTN, the expected behaviour to hear ringback. Ringback is sent to the calling party when the ALERTING Message is received.

You may find in some cases the Telco doesn’t send the ALERTING with Progress Indicator of 8. Therefore when a caller makes a call to the PSTN, the caller does not hear ringback before the call is answered.

The solution is to add the command progress_ind commands and set to digit 8

Dial-peer voice X pots
progress_ind alert enable 8
progress_ind progress enable 8
progress_ind connect enable 8