Ok..with a bit of research I answer my own question and was very nicely surprised.

If you are outside the USA and have wifi calling enabled on your phone (most of the recent Samsung and IPhones) you will receive and can make calls to and from US numbers at no cost.

So Grandma calling you in SXM from Florida will go through as if you are in your home location. No special plan needed. Just be connected to wifi.

This does NOT apply to calling International numbers. So no calling SXM from Florida or calling SXM from SXM. That's what Skype is for...

It works as I found out yesterday by accident...

No cost or special procedure involved..

edited to add: IslandBoi: just saw your post after I posted mine. They seem to agree. Considering all the phone home questions here why was this not mentioned before? It is a no brainer....