What does SIP 503 Service Unavailable mean?


Jim Dell


What is a SIP 503?

A SIP 503 Service Unavailable message means that the call attempt was unsuccessful. However, it’s not an indication of a call failure but rather an instruction to route advance the call to your next available carrier.

Why am I getting a SIP 503?

Although you may receive a SIP 503 message for various reasons, it typically happens if you’re using the Least Cost Routing (LCR) product. If you have LCR, SIP 503 is an expected response and you should failover calls to your next carrier on the route.

Why does using LCR cause a SIP 503?

LCR is one of several basic call termination products offered by Bandwidth. It allows us to target aggressive termination rates by utilizing our entire portfolio of termination providers.

The LCR product has routes that aren’t available to complete at the rate quoted in the customer rate deck. In those cases, Bandwidth will return a SIP 503 and expect you to route advance to another vendor that can complete the call.

Note: Vendors frequently change rate decks and cost structure, so this can oscillate daily. Other concerns, such as upstream vendor capacity, may also have an effect. 

If the LCR model doesn’t fit your needs or you’re unable to route advance on a SIP 503, please reach out to your Account Manage to discuss our other available outbound calling products. Not sure who your Account Manager is? Please open a ticket with your Bandwidth Support Team or hit us up at (855) 864-7776!

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