Mobile providers in the US for backup access

[Dovid Bender <dovid () telecurve com>]

A while ago some people mentioned that some US carriers have basic internet
plans for backup access to their equipment. A few questions:
1) Do they give you a public IP per connection or do you tunnel back to a
central location and then connect via the tunnel?
2) Which carriers offer this and what kind of devices do you use to
connect? Is it simply a GSM card on a "MyFi" like device? We have lots of
Pi's out there that we want backup access to.
3) Can you send off list contacts and pricing that you have gotten in the
past?

TIA.

Dovid