Can anyone recommend any good choices for open source VoIP calling apps for iOS, Android, and Windows that are compatible with Google Voice’s free service? (besides Google’s own “Google Voice”)
Rumor has it four candidates for PC caller apps are: Zoiper (https://www.zoiper.com/), 3CX (https://www.3cx.com/), nextiva (https://www.nextiva.com/), and OpenPhone (https://www.openphone.com/).
You could probably run your own freePBX server if you are feeling giddy.
I always wanted to play around with running my own freePBX server… someday when I have the free time 
My post was regading VoIP calling apps that can utilize an existing free Google Voice number.
I suppose since Google Voice is not truely SIP this might be waste of time.
And most likely any VoIP calling app wil never support Google Voice SMS text messaing.
For any kind of self hosted SIP services you need a trunking service to make use of it unless you just want to experiment with FreePBX and dial each other phones via extensions.