It sounds to me like what you'd like to achieve is tracking the status of a call that is on hold as described in this FAQ.
You can use the StatusCallbackEvent
parameter on outbound calls made via the REST API. If you're indeed using the C# helper library it would look something like this:
var options = new CallOptions();
options.Url = "http://demo.twilio.com/docs/voice.xml";
options.From = "+18668675309";
options.Method = "GET";
options.StatusCallback = "https://www.myapp.com/events";
options.StatusCallbackMethod = "POST";
options.StatusCallbackEvents = new string[] { "initiated", "ringing", "answered", "completed" };
Also, depending on your use case, there is now a dedicated Hold
feature available when using <Conference>
as detailed in this post.