Call charges in UK

I have an 'unlimited' mobile plan in the US and am traveling to the UK. The plan says calls and texts are included when roaming. How are calls charged when in the UK. It looks like my US number will be active and calls are included but what calls are free? Calls to UK numbers? Calls to US numbers? Both?
(International roaming is turned on by support but global data pass is off. I intend to use wifi and another esim only for data)