No Incoming Calls
This section describes a situation where an operator reports that no one can reach them or that incoming calls are not arriving at all. From the user’s perspective, telephony “doesn’t work,” even though the SIP client appears to be registered and functioning normally.
How the Issue Appears
Operators usually describe the problem like this: “Telephony isn’t working”, “I’m not receiving incoming calls”, or “No one can reach me”.
To clarify the situation, ask a follow-up question:
“Do you see the incoming call in the dialer?”
The typical answer is:
“There are no incoming calls in the dialer.”
In other words, the call is not simply unanswered — it never even appears in the interface.
What Is Actually Happening
From a SIP perspective, the situation usually looks like this:
REGISTER → SIP/2.0 200 OK
The registration is successful, and the PBX can see the client.
However, there are no INVITE requests from the PBX in the logs, which means incoming calls are not being delivered.
In simple terms: the client is registered, but the PBX is not sending incoming calls to it.
How to Diagnose
First, confirm the symptoms:
- Incoming calls do not reach the operator.
- The dialer does not show any incoming calls.
- The SIP account is registered.
Next, check the SIP client logs:
- REGISTER is present.
- SIP/2.0 200 OK is present.
- No INVITE requests from the PBX.
If INVITE requests are missing, it means the incoming call never reaches the client.
Possible Causes
The most common causes include:
- The SIP client operating behind a firewall or NAT.
- Network infrastructure issues.
What to Do
On the PBX administrator side
Verify where the INVITE requests were sent and whether they were delivered to the client.
On the support side
Send a request to the PBX support team describing the situation, for example:
“A REGISTER request was sent to the PBX and a SIP/2.0 200 OK response was received. A call was made at [time], but the operator did not receive an INVITE.”
Additionally, it is recommended to:
- check SIP account settings related to firewall traversal (STUN / ICE) and IP rewriting
- verify that multiple SIP clients are not running simultaneously
- ensure that the same SIP account is not configured on multiple devices
Key Point
If there are no incoming calls at all, check the INVITE messages in the logs.
If you see REGISTER with 200 OK but no subsequent INVITE, it means the PBX is not delivering the incoming call to the SIP client.