I'm not a T-Mobile subscriber but I just saw this.
On May 31, every T-Mobile customer’s phone number will be upgraded to DIGITS for free—and customers can get another DIGITS line for just $10 a month with AutoPay. And, for a limited time, T-Mobile ONE Plus with taxes and fees included comes with an extra DIGITS line—at no extra cost.
Up to 5 numbers per device & up to 5 devices per number.
T-Mobile DIGITS 
