If you take a look closer to following table
http://www.ukphoneinfo.com/search/Phones/o...s7_code_76.html . Prefix 07624 is only one, which have dual use eg. Paging & Mobile Services. All remaing are use for Paging Services only. And I believe this is the reason, why Skype considers prefix 07624 as used for Paging Services only and charges calls to this prefix in national rate for UK.