They won't sue!
Let's follow down the chain from bad to worse
1. Internal Sprint collection
2. Account phones cut off
3. Account terminated (if you're in contract you will be final billed with any ETF you may have)
4. Sprint tries to collect by sending you reminders
5. Sprint sends your balance to a credit collection agency
6. Credit collection agency calls you, sends you bills, etc.
7. Unpaid long enough eventually the calls stop but you still get bills
8. Even longer, possible for the debt to be sold off to another collection agency.
9. The debt becomes "zombie" debt where in bouncing around between collection agencies these agencies try to get a hold of you to pay the bill. There is supposed to be a termination of the debt after a certain number of years, but if an agency can prove contact with you then the debt renews. That's why it's called zombie debt.
Somewhere in all of this is the negative report to a credit bureau.
You want to stop this process before number 3 by trying to work with Sprint any way you can.
Note however, that if you have a lot of debts that go all the way to number 9 and your credit is ruined to the point of being on a cash only basis and being only able to qualify for debit cards and so on
well, then number 9 isn't that big of a deal.
Unless you've done something illegal, or written bad checks no one can arrest you or anything like that. Some people get paranoid about their credit rating. Trust me, it sucks having ruined credit, but it's not a life ender. Send the jokers to voicemail enough times and ignore their raving threats (because they can't do anything to you) and it eventually stops.