Our phone number for our Apple ID was changed

What do we do if our apple Id was locked due to security issue , after looking to see why this may have occured We found that our phone number for our apple Id account was changed to a number that looked like this: *** *** **78 which is strange since our phone number ends with this number: *** *** **39 . So we need help in finding out who may have changed the phone number of our apple ID: CherryBlossom (C****@icloud.com), So we’re able to change the password to the account as soon as we’re able to since we can’t sign in to that account at the moment due to apple locking the account due to security reason being the phone number being change by an unknown third source with a phone number ending with 78 (** *** **78), thank you.

[Edited by Moderator]