VOIP for international calls

Being a freelance trip planner, I usually get my work done from the internet. However, occasionally I have to make international call to the destination for further checking and confirmation. To save cost for the international call, I usually make the international call from my office as we have the VoIP phone systems installed. With the VoIP phone system, we are allowed to make international calls with unlimited! Amazing huh!!! However, I still have to make the calls sneaking as I am using t