Creation of the custom routing providers is not so easy and straightforward as creation of custom map tile provider. There are many things you have to do there. You definitely should implement CalculateRouteAsync method. This method should process request parameters and pass them to the Map24 routing service. When the service responds to your request, you should process service data, put it into the RouteResponse object and call protected method RouteProviderBase.OnRoutingCompleted.
Unfortunately we cannot provide you with precise instructions how you should call Map24 routing service as we haven't used it in practice but it looks like you can find more information on Map24 official API here
Generally the implementation of Map24 routing provider is not in our plans for Q3 2010 and it does not seem to be a trivial one. We will make sure it is reconsidered when discussing our future plans, but we cannot commit a specific timeframe at the moment.
All the best,
the Telerik team
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