We use the WebClient class in our WPF implementation of the RadMap control for downloading map tiles. By default it uses proxy settings from the application / system configuration and the Internet Explorer Local Area Network settings (http://msdn.microsoft.com/en-us/library/system.net.webclient.proxy.aspx
So, please check that your proxy server settings allow downloading URLs of Open Street tile images. They are the png-images. Its URL can be started from the following addresses:
The following letters could be used as the prefix: "a", "b", "c", "d", "e" and "f".
the Telerik team