@fify-- I think a combination of emails and telephone calls is the best and most reliable way to communicate in your situation.
I work for a business that has about that many employees and we're all in different states. For important conversations and interviews, we use the phone. But for routine communication such as questions about content or payment, we use email. It's been working fine for us this way.
If the employee number increases, getting back to emails can become more difficult. In that case, you might have to switch to mass emails or teleconferencing. But for now, communication can probably mostly be handled with email.