No! All of our call packages include a certain amount of free minutes to make outbound calls. You will only need additional credit on your account if you exceed your plan's minutes, or to make calls outside of the UK, unless you're on the Enterprise package, in which case, calls to the UK and 54 countries are included in the plan.
Please note that calls to Mobile Virtual Network Operator's are not included in any of our plans. More information on MvNO's can be found here