After getting AMR support in FreeSWITCH I set about creating an IMS Application Server for VoLTE / IMS networks using FreeSWITCH.

So in IMS what is an Application Server? Well, the answer is almost anything that’s not a CSCF.

An Application Server could handle your Voicemail, recorded announcements, a Conference Factory, or help interconnect with other systems (without using a BGCF).

I’ll be using mine as a simple bridge between my SIP network and the IMS core I’ve got for VoLTE, with FreeSWITCH transcoding between AMR to PCMA.

Setting up FreeSWITCH

You’ll need to setup FreeSWITCH as per your needs, so that’s however you want to use it.

This post won’t cover setting up FreeSWITCH, there’s plenty of good resources out there for that.

The only difference is when you install FreeSWITCH, you will want to compile with AMR Support, so that you can interact with mobile phones using the AMR codec, which I’ve documented how to do here.

Setting up your IMS

In order to get calls from the IMS to the Application Server, we need a way of routing the calls to the Application Server.

There are two standards-compliant ways to achieve this,

The first is to use ENUM to route the calls you want to send to the Application Server, to the application server.

If you want to go down that path using Kamailio as your IMS I’ve got a post on that topic here.

But this is a blunt instrument, after all, it’ll only ever be used at the start of the call, what if we want to send it to an AS because a destination can’t be reached and we want to play back a recorded announcement?

Well that’s where iFCs come into the picture. Through the use of Initial Filter Criterias, we’re able to route different types of SIP traffic, requests and responses, based on our needs. Again we can do this in Kamailio, with a little help from an HSS like PyHSS.