Twilio Outgoing calls using Web API

I want to develop Application in C# using Twilio for outgoing calls. I have created Web API which returns XML and deployed it on Azure website. The URL which is generated I passed it in my code for making outgoing calls.

var call = client.InitiateOutboundCall(fromNumber,toNumber,"http://callpoc.azurewebsites.net/api/response");

but I am unable to change the voice of outgoing calls.