Using a SIP Softphone With Your Operator’s Fixed Line IMS

When it comes to SIP, getting things to work is usually not straight forward. The more I was surprised when I recently wanted to connect a SIP software client running on Linux to my fixed line network operator’s IMS system.

There are a number of SIP software clients available and I chose Jitsi for my project. While the software can be installed from the Ubuntu/Debian repositories I instead chose to download the latest .deb installation package from their website.

SIP connectivity the Hard Way

To connect to the IMS system I found out that there is an easy and a hard way depending on the network operator and the DSL/cable modem router at home. The hard way, in my case, is to connect to the IMS system directly. My fixed line network operator in Germany kindly describes how to configure SIP software clients here. SIP REGISTERs worked pretty much out of the box but SIP INVITES were rejected. After I deactivated pretty much all SIP and SDP codec options in Jitsi the INVITE was accepted and I could make incoming and outgoing calls. How nice!

SIP Connectivity the Easy Way

A much simpler way to get things going is if your DSL/cable router at home also implements a SIP server. I have a Fritzbox at home which does just that. So instead of fiddling around with a lot of stuff, getting SIP connectivity to the Fritzbox is as simple as creating a new SIP telephony device in the graphical user interface of the router and to give it a username and password. Together with the IP address of the router that has to be given as the SIP registrar IP address that’s all that is needed to make Jitsi work. 3 parameters and it was working, I could hardly believe it! SIP was never less painful…

Also I was very positively surprised about the speech path delay which is absolutely minimal in both directions, which is often not the case, especially with smartphone software clients.