My child's handset has been broken, and we have no insurance coverage on it nor do we want to pay for a new one just yet. How do we switch the mobile number to another older handset that we still have? Thanks!
Take the SIM out of the broken device and put it in any [T-Mobile branded or compatible unlocked] device.
If the SIM is a different size, you can get a SIM Adapter Kit to fit smaller SIMs to a larger SIM size. If you have a larger SIM and need smaller, go to a T-Mobile store to get a new SIM and transfer the line to the new SIM. There's a fee for a new SIM.