Unless you have used the number with a second account you have, the only thing i can think of is the number was re-issued to you too quickly from when it was used with another person.
Contact Apple about this. This support page has information about Apple support and service ➔ Contact Apple Support - Apple Support If live support options (chat, telephone) are available they will show near the bottom left. Telephone numbers may only work when calling from the same country.
With an iPhone, iPad, or Vision, you may be able to chat using the Apple Support App if it is available in your country —> https://apps.apple.com/app/apple-support/id1130498044