system.net.webexception:the remote server returned an error 417 expectation failed

Question

Hi All,
 
I am getting the following error when I am sending an SMS through http webrequest from my service :
 
system.net.webexception:the remote server returned an error 417 expectation failed. we are using proxyserver.
I have tried changing the machine.config with the following settings, still no success.
 
<system.net>
<settings>
<servicepointmanager expect100continue="false" />
</settings>
</system.net>