Use dashes in US phone numbers. Do not include a country code (+1) before the area code unless you anticipate an international audience. Use ext. for extension if required.
123-123-1234
123-123-1234 ext.12
Use toll-free number, not 800 number.
For phone numbers outside the U.S., match the country's spacing format and always use spaces (not hyphens, periods, or parentheses) to mark number groups.
For example, in France, it's 06 34 27 35 12, while in the UK, it's 07787 123 456 (cell/mobile number) or 0800 123 4567 (landline number).
Do not use plus signs.