Incoming Call Arrives, but Cannot Be Answered
This issue occurs when an incoming call reaches the user, the user presses “Answer”, but the call does not connect. From the user’s perspective, it looks like the button does not work, although the real problem is deeper — at the SIP signaling level.
How the Issue Appears
Operators usually describe the problem in the following way: “The call comes in, but I can’t answer it”, “I press ‘Answer’ and nothing happens”, or “There is an incoming call, but the conversation never starts”.
From the user’s perspective, the situation looks confusing:
- the incoming call is visible
- the interface responds
- the Answer button is pressed
…but the connection is never established. In other words, it seems to work — but it actually doesn’t.
What Is Actually Happening
If you check the SIP client logs, the situation typically looks like this:
- The PBX sends an INVITE request (incoming call).
- The client correctly responds with 180 Ringing.
- When the user presses Answer, the client sends 200 OK.
- ACK from the PBX never arrives.
In the logs, you may also see multiple repeated 200 OK responses, but no ACK afterward.
In simpler terms: the client says “I answered, the call can start now,” but the PBX never confirms it received that response.
How to Diagnose
First, confirm the symptoms:
- The incoming call definitely arrives.
- The Answer button is pressed.
- The connection is not established, and the conversation does not start.
Next, open the SIP client logs and look for the following sequence:
INVITE → 180 Ringing → 200 OK → ACK missing
If 200 OK is present but ACK is missing, the diagnosis is almost obvious.
A strong indicator is when the logs show multiple consecutive 200 OK responses — the client keeps sending them, but never receives confirmation.
What to Do
The most common cause of this issue is network-related — typically involving firewalls, NAT, or network infrastructure misconfiguration.
On the client side
- Check SIP client settings related to NAT.
- Check firewall configuration.
On the PBX side
- Verify whether 180 Ringing from the client reached the PBX.
- Check whether 200 OK was received.
- Confirm whether the PBX sent an ACK.
If the ACK was sent but not received, the next step is to locate where it is being lost in the network.
Key Point
If an incoming call arrives but cannot be answered, always check the ACK.
The presence of 200 OK without ACK almost always indicates a network issue, such as problems with NAT or firewall configuration.