I have Zoiper installed on two iPhone 6's. One works absolutely fine but the other has a strange issue. I can dial any valid number except for UK mobile number (e.g. 07xxx xxx xxx). If I dial a mobile number like this Zoiper just disappears from the screen. The number dialled is not shown in the call history so I guess it didn't even try to dial it. Even if I try to trick the software by dialling the number in International format (e.g. 0044 7xxx xxx xxx) is still stopped straight away and disappears from the screen.
Any help in understanding this would be much appreciated.