Why in movies are all american phone numbers start with 555?

In everymovie

Every american phoneb number always begins with 555.

My mum was wonderingWhy in movies are all american phone numbers start with 555?Because, in real life, there is no 555 exchange. If they used just random numbers, it would probably be someones real phone number. Don't think it would be a problem? Do you know how many people have called 867-5309?!