I am referring to this post by someone else:
THEY are talking about using tethering without paying for it, with a phone that ALREADY has a plan on it. They had no idea what was actually being discussed in this thread and the post has nothing to do with you. I was merely pointing that out to you, sorry i didn't quote the post initially as i was on the phandroid app and unable to quote more than one post at a time.
And yes i merged your 2 threads as they are essentially discussing the same topic as they have the same answer.
A mifi device is a wifi hotspot that you can take anywhere and connect ANY device to it. It is also the only option that i know of other than calling Tmobile or whoever and asking if you can get data only on a phone.
You can, of course, connect a phone with no service to ANY wifi hotspot (your houses, mcdonalds, starbucks, etc...) and use data