You have to pay off 50% of the phone right?
Yes, which basically means upgrading once a year with no additional cost. It works out simple. And with the cheaper plans, it's still cheaper than Sprint, AT&T and Verizon, plus you get insurance. Here's an example for a single line, and a family of 4. With iPhone 6's.
Single line: 1GB of high-speed data
Sprint: 1GB (may incur overage charges if you go over 1GB) $81/month, no upgrade early, insurance
AT&T: 1GB (may incur overage charges if you go over 1GB) $92.49/month, upgrade in 12 months, insurance
Verizon: 1GB (may incur overage charges if you go over 1GB) $112.49/month, upgrade in 12 months, insurance
T-Mobile: (1GB 4G + LTE, unlimited 2G, no overages, insurance) $87.08/month
While T-Mobile may not be the cheapest, they offer international data and text, music streaming without using data, and they don't have overages. And no, I don't work for T-Mobile, I'm just passionate about this stuff
