t-mobile is currently running a promotion where one can get 2 lines of t-mobile one + 10GB of 4G hotspot, double speed of their international roaming. also includes 1Hr of gogo wireless on planes (per flight, unlimited flights). It's really $110 but you get $5 per line bill credit for using autopay
This is a good deal in and of itself. And remember, on t-mobile taxs are now included, so its $100 straight up.
Ending tonight they have get a third line free, this makes it even better.
Further, if you don't really use data on all your lines, you can enable kickback, which gives you $10 back (per paid line, so the free line wont work, so if you only use data on one line, make it the free one). This could bring down your total monthly charges to $80.
Further, if you port in a foreign (i.e. non t-mobile) line onto a new account, you can get a "insider hookup" code (from an employee) that will give you 20% off your t-mobile one lines (i.e. that $80 could now be $64 for 3 lines, and if you have more than 3 t-mobile one lines it will include them all, won't include tablets, hotspots or wearable devices though)
might be too late, but you can call in to port a line and then try to get the hookup code later (people are selling them online, but not cheap, but in the big picture, even a $150 charge could save you lots of money)
