At a glance, my approach would probably be to use a ListView for the phone-numbers, and make the 'add' button add a new empty record to the top of the list. That should provide you with the functionality, but it will still take a fair amount of CSS styling to actually make it look like a regular part of the page.
You can take the Phone Details as a separate entity and associate with the Customer by doing this you can link multiple phone numbers to a single customer. Now using a button microflow persist the data.
I think I might make you confuse. I understand how entity should be associated with but I would like to know how I create the screen like I've explained before. I'm talking about UI & UX on Mendix.
By the way, thank you for your response sir.