In its simplest form, you should enter the number in the same way as somebody would enter it on their mobile phone.
However, this would eliminate non-local callers. When somebody has a foreign plan on their phone, they won’t be able to get through to you. E.g. the expat who still uses her plan from back home.
So, to be completely on the safe side you could include the country code as well so nobody will ever have any issues contacting you.