As far as I know, BT don't include calls to mobiles on their plans ( many other providers do ) , but I think they have a plan were calls to mobiles are cheaper than other plans.
When you have a call plan, calls are not really 'free' they are included , so if you were on the BT Anytime call plan, then the connection charge and pence minute charge for a call to a standard landline number would be included in the plan , so would not cost you anything, they show the charge for those who could make a call outside their plan, for example if you were on Evenings and Weekends, calls are included in these periods , but if you made a call during the daytime during the week Monday to Friday, you would be charged so need to know how much that charge would be.
Some have worked out how many calls you need to make during the day, to make the cost of an Anytime Plan worth it , if you don't make many calls, it may not be worth paying the £ 8-£10 per month ( or whatever it costs ) as the cost of the individual calls made be less than the plan
