Is your mobile number your Apple ID? If yes, see
Use your mobile phone number as your Apple ID https://support.apple.com/en-us/HT207944#change-appleid. See the heading Change your Apple ID to a different mobile phone number.
But, this won't work if a different Apple ID is shown on your phone. If this is what is happening, then you need to get support from you mobile carrier. Still, Apple support might be able to help you. See:
Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362
Apple ID Support https://support.apple.com/apple-id
Apple Support Twitter https://twitter.com/AppleSupport
Apple ID account security for country-specific and region-specific help https://support.apple.com/en-us/HT204169
Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best one.