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VoIP Guide Part 3 - SIP 503 Service Unavailable

We will focus on disconnect codes in VoIP business and the main topic is about most known one: Sip 503 Service Unavailable . Let’s start!

What is the meaning of SIP 503 Service Unavailable ?

Sip 503 service unavailable disconnect code means service is not being provided. In other words, call on the A side can not reach the B side because of some reasons.

Why does SIP 503 Service Unavailable happen?

There are several reasons for SIP 503 code, here are some of them:

Low Capacity

There can be a limit for the traffic which was made by the company that we sent the traffic. Or the route they sent may be working within limits.

For example; If the route has a max of 30 channel instant call capacity, the calls after the 30th one will return with Sip 503 service unavailable code. This generally happens for Ncli routes because for the GSM Gateway, it is impossible to open many ports.

Wrong Number

Some operators return Sip 503 for calling the wrong number. Do not confuse the wrong number with the missing number. If there is a missing number situation it will automatically give address incomplete error code. But these codes can be manipulated so we need to examine the errors in detail.

Route Down

One of the possible reasons for this disconnect code may be the route being down or our numbers may be blocked by the other side. We need to inform the vendor but first for the stability of the traffic, we should direct the traffic to another route.

Code Mismatch

If we send G279 traffic to a provider which doesn’t support G279, there will be code mismatch. Normally the error supposed to be unsupported media type but these errors can be manipulated and we see Sip 503.

Routing Issue

If ours or other side’s routing is empty, the traffic that customers sent will go to VCRS and we return back to the customer as Sip 503.

Wrong Tech Prefix

If you are sending the traffic with a wrong tech prefix number to the other side, then you will receive Sip 503.

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