There are a few ways to go, Marco, depending up how you need the information and what version of FileMaker you are using. These two methods will work no matter the version (7-11):
1) Go to your graph and sort your Addresses relationship (where it is joined to your customer table) in descending order by the PT/AT field. Now your PT record will be your first related. Then, since your perspective from Customers sees only the first record (which is standard FM behavior), it will see PT first.
2) You could also filter your relationship by adding a calculation (c_string_PT, result of text and unstored) to your Customer table and create another table occurrence of Addresses (maybe called PT Addresses). Join by:
Customers::CustomerID = Addresses::CustomerID
Customers::c_string_PT = Addresses::pt/atField