Chat application backed system framework

need team or professional who able to develop real time msg framework back end system for cross platform with Video, Voice, FIle Share and other stuff within the chat system. only need backed team who can build the infrastructure and back end coding. front end developer for app we already have and team working on app using third party api. now we are going to shift the third party api with our own infrastructure for the chat and all other types of real time application.