SIP 100 Trying
What SIP 100 means
Not an error. It confirms your request was received and is being worked on, which stops the sending device retransmitting. If a call log ends at a 100 with nothing after it, the problem is whatever should have followed.
What SIP 100 Trying actually means
The request has been received and the recipient is working on it. Its practical job is to stop retransmissions while a call is being set up, since a device that hears nothing assumes its request was lost and sends it again.
What causes SIP 100 on a business phone system
- Normal call setupEvery call produces one. Its presence means your signalling reached a working server, which is genuinely useful to know.
- A log that stops at 100 means the next step failedWhen a trace shows a 100 and then nothing, the request was accepted and something downstream never answered. That points at the far end rather than at your connection.
How to fix SIP 100
- Treat it as confirmation, not a faultA 100 in a log means the first hop is working.
- If the log ends here, investigate the timeout insteadA call that gets a 100 and no final response is a timeout condition, so the useful codes to read up on are 408 and 504.
Fixing the underlying problem
This page explains the code. These guides walk through the fix in detail.
Questions about SIP 100
Is SIP 100 Trying an error?
No. It confirms a server received your request and is working on it, and it exists to stop your device retransmitting during setup. Every call produces one. What matters is whether something follows it.
Related SIP response codes
Source. SIP 100 Trying is defined in RFC 3261 section 21.1.1 and registered in the IANA SIP Parameters — Response Codes. The causes and fixes above are drawn from our own experience supporting UK business phone systems.