BareSIP

BareSIP is an FOSS app with advanced features that is currently available only for Android. You can download and install the "apk" from its GitHub repository  or from F-Droid; either way, you will have 2 choices: BareSIP and BareSIP+. The only real difference is that BareSIP+ allows for video calls, but this is of interest only if you call other VoIP SIP users having a softphone that allows video calls; BareSIP being voice-only (i.e., simpler) needs less resources to run. If you only call POTS (or people who don't use VoIP SIP) then you would not be able to make a video call anyway.

We will have a more detailed guide shortly.