I find it unhelpful to talk purely in terms of revenue share. It makes a bit more sense to talk in terms of Service Charge.
Calls to 0845 numbers incur a 2p/min Service Charge. The call recipient uses this to pay for the call handling and call routing costs at their end of the call. This means they get to run the 0845 number free of charge by forcing callers to subsidise the running costs. When this extra cost is incurred by callers in contacting a service that is already financed through taxation it has been dubbed "the Telephone Tax".
As the Service Charge only just covers those running costs, most users of 0845 numbers never see a revenue share payment. Very large users might receive half a penny per minute. The attraction is a number that is free to run. The lie that often accompanies the selling of such a number is that it is cheap to call.
They are probably unaware that calls to 0845 numbers can cost up to 12p/min plus a 16p connection fee from landlines and up to 41p/min from mobiles.
I'll guess they either think the calls are charged at "local rate" or, having seen that BT includes some of these calls within their call plans believe that is what happens on other networks.
Ofcom's "unbundled tariffs" with a requirement to declare that 2p/min Service Charge will reveal the true situation with 0845 and other such premium numbers and it will be very a big shock to many.
0845 numbers rarely pay out revenue share and the same is true for some 0843 and 0844 numbers. Those with a Service Charge under about 3p/min also do not pay a revenue share to the called party. Ofcom's unbundled tariffs will show these two types of call to be equivalent, whereas most people would currently, and incorrectly, consider them to be quite different.
A recent NAO report
http://www.nao.org.uk/press-releases/charges-for-customer-telephone-lines-2/ is highly critical of the usage of such numbers for public services as was the recent Public Accounts Committee enquiry
http://www.bbc.co.uk/democracylive/house-of-commons-23934512 which resulted in DWP promising to drop their 0845 numbers in favour of the cheaper 0345 range.
HMRC
http://www.hmrc.gov.uk/news/new-numbers-helplines.htm /
http://www.hmrc.gov.uk/news/new-nums-stamp-taxes.htm /
http://www.hmrc.gov.uk/news/new-tel-numbers.htm and Fife Council
http://fifedirect.org.uk/03 lead the way in moving to 03 numbers.