Hello @Maisam 

Did you change your prepaid plan to abonnement via this page?

There is an option for a new number or keep the same number, by default new number is selected so maybe it went wrong there?

Hi, 

I am sure I choosed to keep my number and I filled my current number in the box. How can I fix this problem? 

I don't want to change my number. However, my prepaid sim has been deactivated. 

@Maisam

The new number could be a temporary number until the old number is transferred. Try inserting the simcard in your phone and check the number, if the simcard is not activated yet please visit a T-Mobile shop so they can activate it right away.