Formats a phone number in the specified format using default rules. Note that this does not
promise to produce a phone number that the user can dial from where they are - although we do
format in either 'national' or 'international' format depending on what the client asks for, we
do not currently support a more abbreviated format, such as for users in the same "area" who
could potentially dial the number without area code. Note that if the phone number has a
country calling code of 0 or an otherwise invalid country calling code, we cannot work out
which formatting rules to apply so we return the national significant number with no formatting
applied.