Das ist nur als Beispiel gedacht, wie so eine Funktion aussehen könnte:

tell application "Contacts"
    activate
    set my_phone_2_check to text returned of (display dialog "Telefonnummer:" default answer "0")
    set allp to people
    set phone_numbers to value of every phone of people
    repeat with i from 1 to count of phone_numbers
        set current_phone_numbers to item i of phone_numbers
        if (count of current_phone_numbers) > 0 then
            repeat with phone_number_2_check in current_phone_numbers
                set phone_number_2_check to phone_number_2_check as text
                if phone_number_2_check starts with "+" then
                    set phone_number_2_check to ("0" & characters 4 through -1 of phone_number_2_check) as text
                end if
                if phone_number_2_check contains " " then
                    set phone_number_2_check to my replace_chars(phone_number_2_check, " ", "")
                end if
                if phone_number_2_check contains "-" then
                    set phone_number_2_check to my replace_chars(phone_number_2_check, "-", "")
                end if
                if phone_number_2_check contains "/" then
                    set phone_number_2_check to my replace_chars(phone_number_2_check, "/", "")
                end if
                if phone_number_2_check = my_phone_2_check then
                    set selection to item i of people
                    error "Die Nummer gibt es schon"
                end if
            end repeat
        end if
    end repeat
end tell
to replace_chars(this_text, search_string, replacement_string)
    try
        if this_text contains the search_string then
            set AppleScript's text item delimiters to the search_string
            set the item_list to every text item of this_text
            set AppleScript's text item delimiters to the replacement_string
            set this_text to the item_list as string
            set AppleScript's text item delimiters to ""
        end if
        return this_text
    on error msg
        error "error on replace_chars" & return & msg
    end try
end replace_chars