connect phone-no to iMessage fails

 I wanted to add my phone no to my apple-id. The iphone 13 mini took the no but stated „can not be used for / connected to iMessage!

The phone no is my“old“ one, that got takwn over from my former provider; the Sim inside is from my actual provider and works fine wirh the old number?

What can I do to receive and send iMessage and not SMS??

Thanks for any Info in advance!