John,
Just a suggestion, but in films, especially American ones, they use 555-###-###, add as many digits or as little as they like. I assume that 555 is an impossible number but movie fans, and I'm sure readers, accept it as valid, as a telephone number in fiction is invalid to begin with.
But there can't be any readers who are going to ring any telephone numbers they see in a book, surely not
You could always try using the real area code and add an extra digit or two on the length of the valid telephone number. Or distract the reader's attention away from the number by giving the sequence of numbers a catchy [ring] to it.
Steven