I use T-Mobile $30 plan (google it to locate it)
100mins, 5gb 4g data. I use Skype for my phone and it uses the data which means I have over 1,000 mins.
T-Mobile in my work location has very high speed data ranging up to 39mb. Inside the building it is pretty much dead, both data and phone. (2 story office building, at my home it works great for phone and data +20mb).
I know 3 others at work with T-Mobile and they experience the same except for one person. He gets 7mb to 10mb inside. He has a Nokia Lumia 1020 GSM unlocked phone from ATT that he put a T-Mobile SIM card in. Great connection. He bought it at Amazon.
I have a Samsung Galaxy S4 that I bought from Amazon that was made for T-Mobile.
I would switch to an ATT unlocked phone if I really need the data for tethering (no way to know which other model except the Nokia). I do not need tethering right now at work and I can make calls outside the building and figure T-Mobile will improve their signal.
The problem is when you search on issues with any phone it may work great in one location and be lousy in another. I searched on that Nokia phone and there are forums where people post their phone reviews.
I may buy a phone model made for ATT but put in the T-Mobile SIM. Maybe ATT has better standard?
Nokia Lumia 1020 phone. (cannot add memory, windows)
http://www.amazon.com/Nokia-1020-RM-877-Unlocked-Windows/dp/B00F590ITY/ref=pd_cp_cps_3
T-Mobile coverage map
https://www.t-mobile.com/coverage/pcc.aspx/
Open Signal - people who have the app create the maps as they drive.
http://opensignal.com/
Do you not have wifi access within the building? If you do, you can just use that for your T-Mobile smartphone.