The R4Telecom mobile access for iPhone works in conjunction with the BT One Voice service as available through BT Global. In order to use the application the end user must be registered with BT. 

This application routes outgoing international calls to a BT One Voice gateway of the country where the call is made. The gateway will connect the call through via the managed BT network. Usage of the app will deliver cost savings for the customer.

It should be noted that the app should be used within a country where the SIM card is registered