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I have the $30 pre-paid plan w/ T-Mobile. The hotspot feature is included in it

Yep. I have the same plan, and it was an unexpected bonus when T-Mobile quietly added it over the summer.

From what I see on my usage screen, the free tethering might be limited to 100 MB on the $30 prepaid plan. Before, I would pay a pro-rated $15/month whenever I needed the mobile hotspot activated. I wonder if that still applies if you go over 100 MB.

I know that with the postpaid plans, you can tether all the way up to the high-speed data limit (up to 5 GB on the unlimited plan). Don't know if you get throttled or cut off if you go over the data cap while tethering.
 
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