When your new emergency phone is installed, TRY IT. When the police officer or other emergency person answers, explain that you are testing a new phone and there is no emergency.
It's a good idea to test once a month or so. Maybe even weekly or daily. It's particularly important to test after a storm when phone wires may have been damaged by falling tree branches.
A home or business phone is typically used several times a day and someone will notice if it is dead, and arrange for repair. Any phone or phone line can malfunction, and it's important that someone check the emergency phone regularly. An emergency phone may never be used, but if you need it, it's vital that it works.
You can put a card next to the phone to show the dates when it was checked, like the cards in elevators.