Every voice engineer eventually learns to think in ladder diagrams. Once you can picture the SIP messages behind a call, most troubleshooting becomes a matter of finding which message is missing or wrong.
The basic call
Caller Callee
|--- INVITE (SDP offer) ------->|
|<-- 100 Trying ---------------|
|<-- 180 Ringing --------------|
|<-- 200 OK (SDP answer) ------|
|--- ACK ---------------------->|
|<========= RTP media ========>|
|--- BYE ---------------------->|
|<-- 200 OK -------------------|The INVITE usually carries an SDP offer describing the caller's IP address, port and codecs. The 200 OK carries the answer. The ACK confirms the three-way handshake, and only then is the dialog fully established.
Where calls usually break
- No 200 OK, or a 4xx/5xx response: a routing, authentication or capability problem. Read the response code and the Reason or Warning header.
- 200 OK but no ACK: often a NAT, firewall or TLS problem on the return path. The call drops after about 32 seconds.
- Call connects but there is one-way or no audio: the signalling succeeded, but the SDP connection address or ports are wrong, or media is blocked.
Our SIP & VoIP Fundamentals course has five guided labs built around exactly these failure patterns.
Go deeper: our SIP & VoIP training covers this topic with hands-on labs.