You might want to try logging in to your Apple ID account online to change your phone number.
Apple ID Log-in: https://appleid.apple.com/
Eg: You may be able to get a verification code sent to an email address you have listed, or if you have another "trusted" device, you could likely get a verification code from that to use:
Get a verification code and sign in with two-factor authentication - Apple Support
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Re: Account recovery access
See: How to use account recovery when you can’t reset your Apple ID password - Apple Support
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If you still need help:
You might want to contact Apple for support and service:
See a list of Apple phone numbers around the world:
Contact Apple for support and service - Apple Support