call_flow_invalid
The flow cannot be written as a valid answering policy for this number (detail names the first reason), or a stored draft no longer validates; save it again.
Problem shape
Every non-2xx response is an RFC 9457 application/problem+json body. The type URI below is stable and machine-matchable, so branch on the final path segment (the code) and treat the status as redundant confirmation. See the error-handling guide.
{
"type": "https://docs.vocapable.com/errors/call_flow_invalid",
"title": "…",
"status": 422,
"detail": "…",
"instance": "/v1/…"
}