You cannot fail to have noticed that in most American films and TV shows, all the phone numbers start 555. This is a reserved number in the North American Numbering Plan. It means that it's a number which will never connect to a real person or service.
So you can avoid the situation where a number is spoken on a show, or in a song, and everyone tries to call it - much to the annoyance of the owner of the number.
That's the US - did you know the UK also has a similar set of reserved numbers?
OfCom have a list of numbers suitable for for use in drama. They're numbers which will never be connected - and therefore are suitable for use as "demo numbers" in an app.
For mobile numbers, the range is 07700 900000 to 900999.
Why Use Them?
I'll tell you a story from waaaaaaay back when I used to work at Vodafone. We'd recently put a site live which asked people for their phone numbers when they registered. Within an hour of go-live, our engineer's phone started ringing. And ringing. And ringing. He'd stuck his number in as an example.
Guess what, people call numbers. People are - basically - stupid.
So, the engineer changed it. To what he thought was a completely random number. That's when we can a call from "upstairs". The "random" number was a string of consecutive numbers - 1234567 etc. That's a "memorable number" and, consequently, was sold for thousands of pounds. The trade in mobile numbers is similar to those of car vanity plates.
So, we had a chap who was spending thousands of pounds with us pissed off because his number was plastered all over the web.
Lesson learned! If you need to use an example number, pick 07700 900123 or similar.