Well actually Kate it's an add a line option on my wife's data plan that she has had for several years. "Even More Plus webConnect� Overage Free Plan" She gets TRUE unlimited (data only-no voice) for 39.99mo. We can add a line up to 5GB for $10 mo. I've already added 1 line for my hotspot. Surprisingly I've been very pleased with Tmo's coverage (and I'm a trucker traveling all over the country). Not always 4G but even 2G is fine for the VOIP app.
My original intent was to deactivate my V-mo phone and carry the Hotspot in my pocket. Didn't take long to realize how impractical it was to carry 2 devices, keeping both charged and turned on.
So I think I'm just gonna get a T-mo compatible phone and add another $10 mo data line. I'll have 10GB a mo for half what I was paying V-mo to throttle me at 2.5 GB.
And of course I"m aware of using the VOIP to conserve minutes not sure about totally replacing voice service. I'm fairly certain T-mo could detect that I've stuck a strictly data sim into a phone via the IMEI #. Just wondering if they might raise a fuss about it?