extra_sessions_not_applicable
The plan includes no concurrent outbound sessions, so the $200/mo add-on has nothing to extend: raised by POST /v1/subscription and PUT /v1/subscription/extra-sessions for a positive target on a plan whose concurrency_limit is 0 - the receptionist class today. Inbound answering capacity is bought as receptionist connections at PUT /v1/subscription/receptionist-connections instead. Keyed on the plan's own included-session count rather than the class name, so the guard reads the same integer the entitlement does. A tenant already holding sessions may always converge back to 0.
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/extra_sessions_not_applicable",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}