Starting today, March 1, T-Mobile is launching a promotion to give customers a free third line. Any new or existing customers with two or more postpaid lines on a single account will be eligible to walk into a T-Mobile store and sign up for a free line with any of the following plans: T-Mobile ONE, Simple Choice, and Simple Choice No Credit plans, as well as business accounts with up to 11 lines. The amount of data allocated to the free line will mimic the line with the smallest amount of data on the account and can be used for a smartphone, tablet, smartwatch, or SyncUp Drive....
Source: http://www.gsmarena.com/tmobile_is_offering_a_free_third_line_for_new_and_existing_customers_for_a_limited_time-news-23734.php
