One of the great advantages of having an app that works on the internet means that it works in exactly the same way, wherever you have access to the internet.
Once our app registers it still thinks it is in the UK and continues to accept calls in the same way without roaming charges*
Similarly, the outbound rates you pay are the same in whichever country you happen to be in.
*Please note, that using 4G/3G data to make and receive calls may end up costing you more in data roaming charges than you are saving on the cost of the call. For safety, only use free Wi-Fi service when overseas to avoid unknown costs.