A lot of attractive applications in addition to manipulation of session related signaling involve specific processing at media level such as playing media, prompting and collecting media from the user, mixing media streams etc. One of the ways of provisioning applications in managed all IP-based multimedia networks, is based on Open Service Access (OSA) service platform. The paper investigates the capabilities for OSA third party control on user interactions in multimedia networks where the session management is based on Session Initiation Protocol (SIP). The focus is on the interoperability between OSA application control on session-related user interactions and media services in SIP networks. OSA user interaction interfaces are mapped onto SIP signaling. The behavior of the OSA gateway is modeled by synchronization of application view on user interaction call and SIP session involving media resources. A formal approach to functional verification of OSA gateway is proposed.