Hi @Francesco Ferrari . Welcome to the forum.

Is this new SIM for your Telfort subscription? Did you reset your phone? If this is not working then a visit to a KPN shop would be the quickest fix.

If the situation is different please fill out your profile and give us a sign after you have done that. 

Thanks for your answer. My subscription of for SIM only. My current phone is a iPhone 10X. I have turned on and off my phone several times, but nothing. Furthermore, when I go to MijnKPN and I try to link the new phone number, I get a message saying that the they don’t recognize the subscription, and that I should verify my information. 
 

I will go to a KPN shot tomorrow, hoping to solve this annoying issue

I have updated my details, providing the new dutch phone number, my client number, and my SIM card number. I would be grateful if you could have a look at what is going wrong

At the moment we have no clue what is going wrong. We have contacted a specialist and will need to wait for his response.

You could try to visit the shop tomorrow but I suspect they wont be able to fix this. I'm not sure about this, though. It's perhaps worth a shot.

We wil report back on monday or tuesday. Hopefully with a fix for your problem.