How To Check If Entity Added To Map

I have a 'user' table, a 'phone number' table and a 'user to phone number map' table. The phone number table stores only unique phone numbers. This way I can take a look at a phone number and see who is using it easily. It is also easy to check if a phone number exists when the user is edited. The question is how I should be checking to see if the phone number should be added to the map. I could loop over the loaded mapped users and check each id. There must be a method that I can use to check like EdmUser.phone_number_map.contains(PhoneNumber). However I cant find anything like this.