There are few reasons why your Voxofon contact has not recieved your free call.
The first step is to make sure that your contact is logged in to their account by the time you make a free call to them. Free call will be received only when user is in logged in state to their Voxofon account.
If that is the case and call was still not received, the second step is to check how strong is your network signal. If your network connection is not strong on your device, then the call might get received with a delay or not received at all. The newtork connection should be strong on your contact device as well, otherwise call might not be received.
The third case is to check if you are calling correct phone number.