As long as you stay around 5gb of data per month you shouldn't have any issues. This is still confusing to me since I use a app that monitors my data usage(I have unlimited text ,5gb 4g data, 100 talk minutes) I use it so I now when my 4g service will kick to 2g ya know. I've been on this plan for a long time and average around 8-10 gb's of data per month. I tether to my PC and sometimes to my ps3 cause apartment has free wifi but its shared also the house has 3 apartments total and we all share it so sometimes with everyone on it at one time it can get slow towards the end of each month. I'm rooted and don't use any special way to tether I just freeze the tether manager apk as I was told to do in the forums as Xda. I've never got any kind of warning for using too much data. Try going to the Google Play store and downloading App Quarantine. Then in app quarantines settings make sure to unchecked hide system services, now freeze the Tether Manager Service. From what I understand if this service is froze T-Mobile cannot tell the difference between tethering and normal data usage.
Hope this helps people having issues with being warned for using to much data
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