Hello, you can have your phone number released at any time in the chat or via the hotline. You can find the contact details for customer service here

https://www.o2online.de/kontakt

If you have an account with o2, you can also take it with you yourself under Contract Manage SIM card and then on Take number with you.

greeting

Hello @Mikine ,

Welcome here to the community.
I see that @0179LoopUP  has already given you the right answer.
Since you have not yet contacted me via the chat or the portal, I have entered your porting request for you in your customer data. 
Please now contact the new provider, who will then apply for the number porting. 

Best regards

Maria