An email came to me from US Mobile today entitled:
"Multi-Network Access is officially launching May 1st"
I used their chat & they confirmed that it is AT&T, VZ & TMob all together.
.
It will be of little use to me 'cuz there's zero VZ or TMob here at home, but if her phone is compatible having both AT&T & VZ on a single phone may serve her well in her travels.
Yeah, I meant to clarify that it is not "AT&T, VZ & TMob all together." It is pick two of the three.
Also that it is not applicable to your grandfathered plan.