The calls are free to 01, 02, 03 & mobile numbers, providing that the duration does not exceed 60 mins, but the boost costs £10 per month I think.
0844 numbers are definitely not included in that boost and their charges are more involved, as there is both an access charge & a service charge for them. This details the charges, with the Access charge being set by TalkTalk at 12.5p, but the service charge is down to the organisation that you are calling.
https://m0.ttxm.co.uk/gfx/help/forms/standard_uk_landline.pdf
According to Ofcom, the service charge can be between 0 & 7p per minute.
