Most often, like others have said, its a curtesy call mixed with sales pitches. But, never ignore the calls assuming they are such. Godaddy is the most prolific registrar out there when it comes to calling users. While failed billing (if you have auto-renew on domains/services) will get an automated email sent to you, they'll often call to remind you of such (especially multiple domains or expensive services like servers or hosting). And, often, if their system is smart enough to red-flag it, they'll call to verify unusual account activity. I usually only transfer one or two domains at a time to a buyer, but Godaddy called very quickly when I once pushed some 50 names at once. They figure better and easier to take the time to verify and clear-up any questions before you come asking after the fact.
