What version of GP are you on?
I looked at the SOP10200 table in GP2010R2 and did not see a field named "Print_Phone_NumberGB"; however, I did not see a field anywhere in the table that did allow nulls. So, as just a guess, and I stress the "just a guess" part try adding the line "customerAddress.Print_Phone_NumberGB = "try different values here" and see if that helps.
Just a shot in the dark,