What should I do if I am harassed by phone calls and text messages?

You can report it. The police will accept it.

Inquiring by number can only be an internal matter of operators. You can have someone check it out. (Not allowed in principle)

But you can only see the registration information of the number. I can't see anything else

Personally, I think I can change to a newer mobile phone (the worst is Saipan, the best is unlimited. Most smartphones support blacklists. )