Unable to receive verification code on my phone number linked to Apple Account

My MacBook keeps asking if the phone number linked to my account is still mine. It’s not, so I click "Change number." But when I enter my current phone number, I just don’t receive anything. I’ve been trying for several days, and it never works—whether I try from my MacBook, iPhone, or even on the website. After a few attempts, I get the message: "Apple verification codes can’t be sent to this phone number at this time."

This number also doesn’t work with iMessage. When I try to enable it in "Send & Receive," it just keeps loading endlessly. I’ve already checked with my service provider, and there’s no issue on their end.

I think the number might be blocked—either by me or by the previous owner—but I have no idea what to do about it. I’ve tried contacting Apple Support through the app several times, but I keep getting a message that the chat is too busy.

[Re-Titled by Moderator]