From Kolmisoft Wiki
If user complains that he does not hear ringing on his device when call is in progress:
1. Do NOT use IAX2 - switch to SIP
2. Configure your phone with same settings as your users and dial same number
If you can hear ringing - advice user to change its device
More detailed troubleshooting:
1. Make call from your Device - get sip debug
2. Make call from User's Device - get sip debug
3. Compare debugs, if packets are the same - problem is in User's Device
4. Send sip debugs with comments to the Client and advice to change User's Device
3. Enable sip debug and full log and look for similar message:
<--- SIP read from 220.127.116.11:64439 ---> SIP/2.0 180 Ringing Via: SIP/2.0/UDP 18.104.22.168:5060;branch=z9hG4bK78d3ff31;rport From: "37068111854" <sip:email@example.com>;tag=as1f2d0c9d To: <sip:firstname.lastname@example.org:5060>;tag=as1f2d0c9d Contact: <sip:email@example.com:5060> Call-ID: firstname.lastname@example.org CSeq: 102 INVITE User-Agent: X-PRO build 1082 Content-Length: 0
If such message exists that means that callee responds properly and problem is at the other end.
4. If you are dialing over some Provider to DID in MOR and get no ringing, try to change Provider. If it works - that means problem is with Provider #1.
If from the provider side no SIP 180 Ringing is received - this means no ringing is generated.
MOR Billing does not influence this in any way. Because MOR Billing is not generating ringing at all - it just passes it on if it receives it.
The provider for different numbers/destinations can use different routes - this means one route
can be good quality and you have ringing, another one has bad route and no ringing is generated.
5. Make a SIP debug:
If you can see that Provider sends 183 Session in progress to MOR, but MOR sent 180 Ringing to client, please set "Progressin-band" option to "No" on Device and Provider settings.
6. Use Fake Ring as last resort