Invalid phone number

I am trying to sign up to play Warzone, but it keeps asking me for my phone number. I am unable to change the country code for my phone number (it’s like greyed out, cannot modify it) and in whatever form I type my phone number, it says that it’s invalid.

There are quite a few restrictions as to the types of numbers the system will accept. You basically need a contract paid cell plan registered in the country you live in - and that is on your account.

Here is the article on it
https://us.battle.net/support/en/article/202594

You can change your country, but may need to submit proof of residency to do it.