WebService calls can be made from the UpdatePanel which has the advantage of reducing the viewstate. These calls can be made static.
If using the Telerik controls then watch out for a conflict between them and ASP.NET Ajax as there are some incompatibilities.
To use a WebService call in Ajax you need to decorate the call with the [ScriptService] and [ScriptMethod] attributes.
If you put the ScriptManager reference (required for all ASP.NET Ajax pages) on a master page then you need to use a ScriptManagerProxy reference in the control that you're building. There are a number of arguments (attributes) that can be given to ScriptManager that I haven't used before that need further investigating.
The EnablePageMethods=true is a ScriptManager attribute that makes the page act like a web service.