I needed to develop a softphone with call holding feature and I used this sample program http://www.voip-sip-sdk.com/p_353-hold-call-voip.html for this purpose provided by Ozeki VoIP SIP SDK. I could build this call holding function easily as they provided a very useful source code explanation as well.
If you are interested in VoIP communication technology, on the website you can find many useful sample programs and source codes too. ;)

Best regards,