SIP Response Codes Explained

Why don’t we start with some trivia?
Did you know that SIP response codes are based on HTTP/1.1 response 404-not-foundcodes?

Actually, SIP response codes are the extent of the original HTTP ones and even define a new class with more response codes. I’ll let you know which one when we get to it.

That said, not all of the HTTP codes are relevant and mapped to SIP response codes, so if you know some HTTP, don’t expect to find them all in this list.
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Cisco SIP Gateway configuration: The Ultimate Guide

Connecting the Cisco IOS Voice Gateway to CUCM via SIP has been the preferred way to do it in the past couple of years.

The slowly dying H323 protocol (ISDN based) is not being developed anymore while SIP (HTTP based) became the industry standard for VoIP. So whenever PSTN connection is implemented via an IOS Voice Gateway, the choice should be really between SIP & MGCP.

This is how it’s done, step by step, using SIP.

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CUCM SIP Call Flow Troubleshooting

SIP Call FlowThere is no way around it,
finding the SIP call flow is the first thing you have to do when you are facing a SIP call failure.

Because first of all, you have to understand whether this is a call routing problem or signaling/media compatibility issue.
SIP call flow helps you understand just that, and in a lot of cases, you can pinpoint the problem just from looking at the SIP call flow.

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