Sorry for the late reply. Doesn't the RIA service return the whole collection of items? If it does return the whole set of items, then the total number of pages will correspond to the number of items in the collection. If the RIA service returns only part of the total number of items on the server, then no items control is able to know how many items there are actually stored on the server.
If I have not understood your question correctly, could you please elaborate a little bit more on the scenario you are trying to achieve.
the Telerik team