An oft-used example phone number is the numeric progression.
It's widely understood that this doesn't mean you should call that number -- it's an example. Additionally, according to the North American Numbering Plan, it's an invalid number -- area codes can't start with 1.
Alternatively, if you don't need digits, the pound sign, all zeroes, or some other pattern could be used as well.
(Note that triple 0 is the Australian emergency number, so use all 0's with caution. Thanks @Gary for the heads up.)
If you actually need something that's designated by regulatory standards: Wikipedia: Fictitious Telephone Number has numbers designated by country. For example, in the US, 555 numbers are almost universally fake, although only the 555-01xx series are officially reserved, according to Wikipedia. The UK's Office of Communications has added many numbers to a list (e.g. 0306 999 0xxx) as well, in addition to many other countries.