control does not have modal behavior by default, but in case that you set the Modal
property to "true" then you cannot interact with the parent page. In your case, I suppose that you used that property with value "true".
In case that you point the predefined dialogs(radalert...), you need to use the following approach (those dialogs are modal by default) :
|var oAlert = radalert("Rad Alert");
I hope this information helps.
the Telerik team