It is normally the routine of a knock on the door by a friendly “salesman” stating they were in your neighbourhood doing some work and that a neighbour had suggested they do the same “free check” on your hot water service.
This unsolicited pitch or a variation of it is a signature of rip-offs who prey on gullible homeowners, especially the elderly. They commonly show a great deal of empathy and concern and are only doing the “free check” of your hot water service as your neighbour had already had theirs repaired and were concerned for you.
In the photo on the left the anode on the left was replaced by Whywait Plumbing with the one on the right with the differences between old and new perfectly illustrated.
Take it from me knocking on your door and cold-calling is not a professional or viable way to market or operate a plumbing business. And yes to work on your hot water service you must be a licensed plumber. In all likelihood the person knocking on your door is not a licensed plumber or a licensed plumbing contractor and in our experience they are ordinary old con artists who have no experience or knowledge in servicing a hot water service.
Yes your sacrificial anode on your hot water service should be checked at least every 5 years. But if it has never been checked and your hot water service is over 7 years old you are wasting your time replacing it as the damage has already been done to your steel storage tank.