On July 1st 2015, Ofcom rules came into place designed to make calls to 08 numbers easier to understand. Unfortunately, the opposite occurred. Ofcom's change meant the cost of the call was defined by two parties: the provider of your phone service (the Access Provider) and the provider of the person you are calling (Service Provider).
The net result is that calling an 08 number can be charged at any cost; a cost which, moreover, can vary from one day to another, although there is a small proviso that the party responsible for the service charge need to advertise the cost of their part of the call. Companies like Yay have therefore had to take a cautious view to calling 08 numbers and consequently can no longer take the risk of including 08 numbers in all call plans.
We strongly recommend checking the 08 number you wish to dial in our call checker before making a call to establish the current rate for the exact 08 number you wish to dial. This is incredibly frustrating, and consequently has caused businesses to move to 03 prefix numbers, which now have much clearer Ofcom guidance, and as a result, we include 03 numbers in our call plans. The site Moneysavingexpert.com has done a great job of documenting the actual issue that resulted from the "simplification" of the rules by Ofcom.