Cannot verify phone number

You wrote - I do not recall linking my phone number with any other account. Is there a way to remove my number from the unknown account it is linked up with so that I may add my number to my actual account.

To enter an Apple ID account you must have the questions and answers written with you , once you enter the account by filling up the details go to accounts and click on edit .

Scroll down in the window and to remove an old mobile number click on cross sign , to add a new mobile number click on add more and check the small circle of phone number and fill up the details - click on continue and finally save it .

This method applies for both old apple id account and new account , condition you need Apple ID questions and answers to enter it .

Whoever is the owner of the another account will reset it see this article If you forgot your Apple ID password - Apple Support

Or if I he / she knows the question and answers for Apple ID that person can enter the apple id account , then he can delete your mobile number from the account .

You can't enter his account without his permissions ( and if luckily he reveals the data to you ) , then this problem can be solved .

Also read this article https://support.apple.com/en-in/HT204074