Hello @Bruno77

If you want to take your cell phone number with you to another provider, please cancel your prepaid contract. You can do this by phone. A note for you: When canceling, please indicate that you intend to take the phone number with you to another provider. After receipt of your cancellation, we will terminate the prepaid contract with a notice period of 14 working days. As soon as you have received your cancellation confirmation, you can ask your new provider to port your number.

You can have the remaining balance paid out to you

German law offers two possibilities of porting a mobile number:

1. during a running contract, i.e. the contract will continue once the porting is done and your old contract will get a new number while the "old" number is then with the new provider; no cancellation is required here (but of course possible once the porting has been successfully accomplished)
   for this variant you need to get in touch with customer service and request that they mark your prepaid contract for "Opt-In"
   for this variant you need to tell the new provider that you want to port "sofort" (immediate)
2. at the end of a contract, that means that you normally proceed best as described by @Marcel2605 

Once the contract is done you can get the remaining money back from Telekom, there is a form for that (note: only the money in your prepaid account that you explicitely have topped up will be given back, so no bonus will be given back)