SIP 503 Service Unavailable (and 500/502): Capacity, Routing and Account Causes

A 503 means a server could not handle the call right now. On a termination trunk the usual causes are concurrency or CPS limits, no available route for the prefix, or account status.

What the codes mean (general SIP)

  • 500 Server Internal Error — the server failed unexpectedly while handling the request.
  • 502 Bad Gateway — a downstream server returned an invalid response.
  • 503 Service Unavailable — the server is temporarily unable to process the call: overloaded, in maintenance, or with no available route.

Common causes on termination trunks

  • Concurrent-call or CPS limit reached — 503s appear in bursts at campaign start or peak.
  • No route available for the dialled prefix at that moment.
  • Account out of balance or temporarily suspended.
  • Upstream carrier outage affecting a destination.

How to diagnose

  • Correlate 503 timestamps with your dialer's active-call count and call-attempt rate.
  • Check whether all destinations fail or only some: all = account/capacity/connectivity, some = route specific.
  • Check any Retry-After or Reason header in the response.
  • Lower the dialer's pacing and confirm the 503s disappear.

At CallCenterBoost

Concurrency and CPS for a service are agreed during provisioning, not published as a guaranteed figure. If you need more capacity, request it in a VoIP Route ticket before raising dialer pacing. Wallet balance and service status are visible in the client portal; current platform incidents are listed on the Service Status page.

Related pages

WhatsApp, Telegram and email remain available for pre-sales and general enquiries. Registered customers use the Client Support Portal ticketing system as their primary, trackable support method. Payments are accepted in cryptocurrency only.