I have long wondered whether I should perform a Nummernportionierung at such a cheap offer. But I dared and can only say that everything worked smoothly. The number portability from my previous provider to DeutschlandSIM was so light and to the day I had the "old" number to my new SIM. Everything was so well described, that even a complete novice like me could not be far wrong. I can only recommend DeutschlandSIM.