The request is pending behind another request already in progress for the same dialog (Request Pending).
In the status line of a SIP response, often with Reason and Warning headers that say more.
Got a trace with SIP 491 Request Pending? Drop it in the analyzer: it finds the step that really failed first, which node sent the code and what to ask the other side. Captures, test-system PDFs and vendor exports. 5 free analyses a month, no sign-up.
Analyze a trace free