I'm grandfathered on a really good unlimited Tmobile plan for a really cheap price and use one phone for our home internet (thank you Binge On!) and one as my primary line.
I like having the Sprint line as an extra phone, however, and will probably find a way to keep it after the free year is up. For all the bad press Sprint gets, I've actually had nothing but positive experiences with it.
I had my brother switch from Verizon to Sprint's free year because, where he lives, Sprint gets the best signal, by far.
Assuming that Sprint offers some kind of 4 or 5 lines for $100 plan I'll probably throw together an ad hoc "family" that will include my brother, me, and some friends.
I have plenty of time to plan, though, since my year just started last week.
"Your Sprint account has unused credits that can occur when service credits and overpayments applied to your account exceed your monthly charges:
Service credits can only apply to your monthly service charges. They can’t be applied to equipment lease or loan charges.
Overpayments will automatically be applied to your previous balance; monthly loan and lease equipment charges; and service charges.
The Total Amount Due (including monthly equipment lease or loan charges) must be paid, even if you have unused credits.
Today, the unused credits on your bill are made up of combined service credits and overpayments."
What's your T-Mobile plan? I have really great plan with T-Mobile too. I got it via the migration a few years back (18 lines unlimited for ~$250/mo.). Select Choice.