The app and service is designed to call from your country to other countries, it can be used from abroad a well however please consider following:
We have three calling methods: Local access, WiFi and Callbacks.
The Local access calls use minutes from your plan, you are billed from your carrier for this call plus the Voxofon rate.
The WiFi calls only use Internet data, plus the Voxofon credit.
The Callbacks only use Voxofon credit and a small amount of data to establish the call, but is more expensive, your mobile plan minutes are not used.
If you use the Access number method from abroad, the nearest access number will not be local for your carrier, making you incur in roaming charges, although the app detects if you are on roaming and asks to confirm the call to the access number always.
The best way to avoid roaming usage is through the WiFi method, as only internet is used.
The Callback may work only if your carrier does not charge you for incoming calls when you are abroad.