a Polycom rep says the only way to call in the app is IP dialing only (basic mode), or with alias or IP based (pro mode) - custom URL scheme to open the app and direct a specific call is not supported as of June 2013
see Polycom rep answer identical question here on their forums - http://community.polycom.com/t5/RealPresence-Mobile/Is-it-possible-for-the-RealPresence-Mobile-iOS-to-dial-URL/td-p/13480