Audio - What should I do if I can't hear ringback tone?

Please go to GSM Outgoing groups and set "Delay for CONNECT" = 0 and "Delay for ALERTING"=4 in all groups.

If you want to hear the tone and voice messages (like "We are sorry, the called party is not available.") from GSM network then verify that   "Send 180 Ringing instead of 183 Session Progress" is disabled in VoIP parameters and that  "Generate virtual tone" is disabled in GSM outgoing groups.

If you want to overwrite tones and voice messages coming from GSM network with generated tone by gateway then set appropriate "Ring tone" in GSM Basic Parameters and select "Generate virtual tone" in GSM Outgoing groups. Verify that   "Send 180 Ringing instead of 183 Session Progress" is disabled in VoIP parameters.

If you want to overwrite tones and voice messages from GSM network by ring-back tone from IP PBX/IP Phones then enable "Send 180 Ringing instead of 183 Session Progress" in VoIP parameters.