No Ringback Tone, Call Is Not Recived
This issue occurs when a call appears to start, but nothing happens afterward: there are no rings, and the call does not reach the client.
How the Issue Appears
Operators usually describe the problem as: “Telephony is not working”, “I can’t reach the client”, or “There is no ringing”. In practice, this is essentially a “call into nowhere”: the call attempt is initiated, but the connection is never established.
How to Diagnose
The first step is to determine whether the call reaches the destination number and whether a ringback tone is heard. The easiest way to check this is to ask the operator to call the specialist’s mobile phone (for example, yours). If the call does not arrive and no ringback tone is heard, the issue is confirmed.
Next, review the logs (see the relevant article for instructions on how to check them). Typically, the logs show that the client sends an INVITE request, then another, and another — but no response is received from the PBX. In other words, the requests are being sent, but nothing responds to them.
Possible Causes
In most cases, the problem is located in one of the following areas:
- SIP client operating behind NAT or a firewall that blocks or filters traffic.
- Incorrect SIP account configuration.
- PBX-side issues or restrictions, causing requests to be dropped or never reach the destination.
Solution
First, check the SIP client settings related to NAT and firewall, such as IP rewriting and STUN configuration.
At the same time, the PBX administrators should verify whether INVITE requests from this client are reaching the PBX. If the requests do not appear on the PBX, they are likely being lost somewhere along the network path. In this case, a network administrator or provider may need to investigate.
How to Formulate a Technical Request
When escalating the issue, provide a clear technical description, for example:
“At 14:52:12 an INVITE request is sent to the PBX, but no response is received.”
This helps the colleagues responsible for the PBX identify and localize the problem much faster.