I’ve used a few different Android SIP clients as extensions on FreePBX and my current favourite is CSipSimple
Installation and setup is straight forward. There are several built in configuration profiles for call providers, or you can choose advanced and enter your FreePBX server details to use CSipSimple as a FreePBX extension.
CSipSimple integrates well with Android, using the inbuilt address book. When you make a call it will automatically launch and register with your FreePBX server, allowing you to choose between a regular GSM call or a SIP call.
Call quality during my testing was good, although as always when using WiFi or 3G call quality will vary depending on your connection. Also if you are going to be using 3G it’s definitely a good idea to install an application to monitor your bandwidth usage. There are options for echo cancellation and different codec selection.
You definitely wouldn’t want to use your Android phone as your main SIP device but for occasional calls while on the move (especially for international calls where your would not want to use your regular mobile call plan minutes) it can work very well.
I did a small amount of testing with inbound calls, which worked well, but I didn’t test how much having CSipSimple running all the time would affect battery life.
It’s free and easy to set up – why not try it out!