Applescript - Contacts: Why can I get addresses and emails from a contact but not phones
This should work for you...

tell application "Contacts"
    launch
    delay 1
    tell its person 301
        try
            set thePhoneNumber to value of phones
        end try
    end tell
end tell


This also works for me...

tell application "Contacts"
    set thePhoneNumber to value of phones of its person 301
end tell


You can also try running this following code

tell application "Contacts"
    set thePhoneNumber to value of phones of people
end tell


Then you will go to item 301 of that result and if there is no phone number... That may be why you are getting the error.