There is no charge for using the Wifi / 4G facility, but if you make a call to a service where the provider has set a charge, costs will be incurred, just as it would if the call was made over the cellular network.
However, you can manage your costs by setting a Spend Control¹, through your my.plan dashboard.
Go to: CONTROLS > SPEND CONTROLS > CALL LIMITS > UK/EU VOICE > [select user] > SET UP
Alternatively, set an Ofcom Total Spend Cap². Go to:
CONTROLS > SPEND CONTROLS > OFCOM TOTAL SPEND CAP > [select user] > SET UP
¹ available to Gold & Platinum account users and Essential account users via an upgrade
² available to new customers from 1st October 2018 and existing customer who renew from the same date