I believe I have found an acceptable solution for my Sailfish device. I’m switching to a Tolla data only SIM an have installed and tested the TextFree android app from Aptoide. To get unlimited use I had to created a Google account and activate the subscription from an android device that had downloaded the app but once done it appears to work OK.
Textnow on the other hand will only run on unrooted, android native devices so its out of the question.
Skype is probably what I would have tried next.
Viber allows out calling but is per minute.
Silent Phone offers out calling but for only very limited number of minutes.
I’d be interested to hear from anyone who has successfully implemented the sip solution. Most of those providers I have looked up appear to be oriented to multiline business accounts.
If this continues to work well in real life, I’ll end up saving $50 a month over my T-Mobile unlimited postpaid plan…so I suppose I should thank them.