I found the problem :

The reason is that the method name at the proxy class MUST match the
webmethod name, so:

 [SoapDocumentMethodAttribute("ns/B")...]
 public int B()
 {
      object[] results = this.Invoke("B"...)
 }

will work, but

 [SoapDocumentMethodAttribute("ns/B")...]
 public int BBB ()
 {
      object[] results = this.Invoke("B"...)
 }

wont (note the mismatch between web method B, and proxy class method
BBB), throwing "Web Service method name is not valid".

This triggers the following two questions:

1) is there any way to circumvent this default behaviour?
2) do you know where can I find good documentation explaining all the
plumbing involved inside the WS black box?

Thxs