To make international calls simply dial 001, followed by the country code and then phone number. For example to call a UK number you could dial: 001 44 1529 30683.
You can also set your default country on your RingRoost PBX. This lets RingRoost know to prefix all calls with the country code. For example if you set your PBX country code to UK. To reach the number in the above example you would now only need to dial: 1529 30683