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RFC 2606 standard reserves the domain names example.org, example.net and example.com for the purpose of being used as examples in documentation.
What is an equivalent for a phone number (including country code) that can be used as an example, e.g. for giving users an example in what format to input phone numbers?
In the best case, it would be a dummy number designated by the relevant standards to be an example phone number, and which would not be attributed to any real subscriber.
The North American Numbering Plan reserves 555-01 numbers for fictitious purposes. If you want an example Seattle number, for example, +1 206 555 0100 - +1 206 555 0199 would do.
In the United Kingdom, Ofcom, the regulator, has set aside numbers for this purpose. For example, if you want a Leeds number, +44 113 496 0000 - +44 113 496 0999 may be used.
I'm sure other countries will have similar things, but I doubt there's one consistent rule across all countries.
Australia lists ranges for premium, subscriber, toll free and local rate numbers.
Ireland - look for "drama use", which currently lists 020 91X XXXX as the only fictional range.
There's only one real standard for representing telephone numbers - E.164 - so from the perspective of storing a fictitious number, spaces don't matter - +44 113 496 0000 is the same number as +441134960000. But from the perspective of rendering a number to a user, there isn't a global standard, and even within a country there isn't usually a standard. In the US no one would give out their number as +14255550123, they'd use (425) 555-0123, or 425-555-0123, or 425 555 0123. Within the UK, the 3-3-4 (+44 113 496 0000) format is just one of them. Some numbers are 2-4-4 (+44 20 7946 0000), and many numbers are a 4-6 pattern (+44 1632 960999). See this Wikipedia article for more.
An oft-used example phone number is the numeric progression.
(123) 456-7890
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.
(###) ###-####
(000) 000-0000
(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.
(123) 456-7890 looks like an invalid number to a human, it only looks so at second glance. Also, any automated system won't recognize it as invalid. It is entirely possible for a telco provider to assign this number to someone. There are actually people who suffer a lot from this. I once watched a report about the poor inhabitants of a house with the address "123 Main Street" in the post area code "12345" and the huge pile of junk mail they receive every day. Not so funny when they come from debt collectors or lawyers.
There aren't any international standards governing phone numbers, except for the internationally agreed country codes.
Each country has its own conventions regarding leading zeros, parentheses, and spacing.
The closest you'll get to an internationally acceptable number format would be:
+0011122223333
Where + denotes the international dialing prefix for your location, 00 the country code (can be fewer/more digits), 111 the 'area' code (can be fewer/more digits), etc.
If it's client-side phone number validation that you want, then check out this International Telephone Input jQuery plugin. It'll give you an idea of the complexity of the problem, if nothing else.
+230 5 nnn nnnn) work?