My Phone Number (United States) cannot be used to register on WhatsApp

Hello,

I bought the MySudoGo subscriptio in order to use a different phone number when using WhatsApp. When wanting to register using the MySuso number +1 (646) 499-xxxx it says that this phone number is not valid for the country United States.

Am I doing something wrong? Or is the phone number maybe not a valid phone number?

Regards
Nicolas