why STS does not get invoked with Dispatch and Provider?

Hi,

    I have a STS, a client and a service. If the service is implemented as SEI, the STS can get invoked without any problem. However, if the service is implemented as a service provider and is invoked by the client with Dispatch API, the STS does not get invoked at all. There is no difference of the security configuration between these two cases. I am wondering whether this is because the Dispatch API does not support STS or something I did wrong.

   Can anyone shed light on this issue?

   Thanks in advance.

   Lisa