Problem:
When the gateway restarts, loadActiveCalls() reloads non-terminal calls from
calls.jsonl. However, these calls may have already ended (e.g., Twilio timed
them out, or webhook couldn't reach local URL) and are now stale. This causes
the concurrent call limit to be reached with phantom calls.
Solution:
- Add getCallStatus() method to VoiceCallProvider interface
- Implement getCallStatus() for all providers (Twilio, Plivo, Telnyx, Mock)
- On load, verify each non-terminal call with the provider before adding to activeCalls
- Skip calls that the provider reports as terminal (completed, failed, etc.)
- Also skip calls older than maxDurationSeconds as a fallback
This is an improvement over PR #2810 which only uses time-based cleanup.
By querying the provider, we can accurately determine if a call is still active.
