BareSIP

BareSIP is an open source app with advanced features that is entirely free to download, use and install, and it is currently available only for Android. You can download and install the "apk" from its GitHub repository. To do so, you will have to allow downloading and installing apk files, which is a security risk, but it is the only way to get this app on your phone. Make sure you save it to internal storage and not the an external card (if you have one), and after install, revert back to disallowing apk file installations. Installing from GitHub (also called "installing from source") might not help with app updates.

You can also install BareSIP from the F-Droid repository, which is an app "store" similar to those owned by Google or Apple, except that it is dedicated solely to open source software (aka OSS).  You install F-Droid by downloading the F-Droid apk from the official website, and by allowing the installation of apk files. To complete the install, it is best to download to internal storage, allow installation of apk files, complete F-Droid installation, allow installation of apk files from F-Droid, then go back to preventing apk installs from the browser. Needless to say, you will encounter a number of warnings that you will have to override, so if you are unsure or concerned, this might not be your best bet. Additionally, some apps (such as Open Camera, Termux, Linphone) can be found on both F-Droid and Google Play, and although both app stores will alert you when there is a new version, you must remember to continue to update them only from the store you first installed from. 

If you choose F-Droid, you can search BareSIP in the store app, and you will find two kinds: BareSIP and BareSIP+. The only 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. If you only call POTS (or people who don't use VoIP SIP) then you wouldn't be able to make a video call anyway.

We will have a more detailed guide shortly.