I contacted ting's customer support (my carrier, and they use Sprint's network), and this was the answer given:
"Thanks for writing to us about this, and I apologize about the delay in response! Given what you've explained about the calls bouncing, I think we may need to take a look at the programming info of the phone. It's pretty easy, and I've provided instructions below:
Tap phone icon and enter: # # 270471 #
Tap Edit
Tap MDN
Enter 6507016076
Tap OK
Tap MSID
Enter 6505200508
Tap OK
Tap Back
Tap Done
If phone does not reboot on its own, reboot it now.
Update Profile after device reboots.
To update the profile:
Tap Apps
Settings
From 4 settings menu options at the top of the screen, tap General
System Update
Update Profile
Tap OK when complete
This should do the trick, but be sure to write back to me so that we can keep in touch should you need help with anything else.
Enjoy your evening!"
I've done the edit as described, and we'll see if that helps. But then I would have to turn of the lockscreen to see if it works. I'm assuming that set of instructions is specific to ting, so it's probably a different set of numbers to input for another carrier?