I have DIGITS with Paired Data on my Simple Choice Plan.
When I originally read about Digits with Paired Data, I thought it will allow me to get rid of my home phone line which is a VoIP line. That was also TMobile's vision in coming up with this product, as I got from the advertisements.
But after using the product for a day, I have hit upon one shortcoming which severely restricts this vision from becoming a reality. It turns out that you can place international calls [as a part of your plan] only from your primary phone. It means that if different people are in different parts of the home, they must find who has the primary phone and use it to make an international call. That introduces a lot of friction in user experience with this product and makes me a less-than-fully-satisfied-user.
What is worse is that it is actually possible to make an international call on any phone. It is just that you will get billed for those calls at the much expensive out-of-plan rates. So imagine you are making these calls by mistake and then at the end of the month, you get a nasty sticker shock on seeing the bill. Exactly the kind of situation that TMobile is renowned for avoiding [which is what makes this shortcoming so glaring].
I know that DIGITS is a work in progress and is becoming better day by day. I would like to request TMobile to remove this restriction in the future. TMobile could always use some type of technological solution [like disallowing phone calls in parallel] to make sure no one misuses DIGITS but having this type of restriction at all time on all DIGITS users kind of defeats the purpose of DIGITS.
Full disclosure - I have not actually tried to test this and am only going by what was informed to me by customer reps that I talked with over the phone. From what I know, International calling can be very expensive unless you have it included as a part of your plan. As a part of the plan, it can be very inexpensive.
In fact after being told this problem, I am scared to make International calls even from Primary number fearing that by mistake I may have secondary phone in hand and not realize that is the case.