The "International Telecommunications Union" (www.itu-t.org), a subcommittee
working under United Nations, stipulates that phone numbers are to be written
as
  +15551234567
or when additional clarity is preferred it is alternatively all right to use
delimiters space or hyphen
  +1 555 1234567
  +1-555-1234567

Parenthesis is not an advocated delimiter in international phone numbers
even though it may be used in national phone numbers in some countries.

Reference documents:
ITU-T E.123 (notation for national and international telephone numbers,
e-mail addresses and web addresses)
ITU-T E.164 (the international public telecommunication numbering plan)

However I have nothing against the ability to use parentheses, but I think
that phone number formatting should be a user preference since clearly
different countries prefer to write their national numbers in different ways.
Preferably this could be controlled by the settings in the Control Panel's
Regional and Language Options in Windows.