For freelancers always on the move, having software to support international efficiency is critical. To start covering the bases, start here.
I can vouch for Slack, Trello and Upwork. As far as international calling, if it is to the U.S., Google Hangouts is great. I also use Facebook Messenger for calling, a lot, and the quality is good. I am excited about WorkFrom to find the best places to work and I will try that out next.