The invention relates to a calling system, in particular to a pure IP calling system based on a cloud computing platform and an IMS network architecture. The calling system aims at solving the problems in achieving calling center services on a CS domain that a relaying circuit is long, a computer room occupies a large space, and a system is poor in expansion capacity. B public mobile phones and C public mobile phones in the calling center system are all connected to a mobile CM-IMS net, a calling center server, a service system server and A system switches are all connected to the mobile CM-IMS net through a transmission bus, the calling center server, the service system server and the A system switches are all connected to an IP transmission network, and D far end seat servers are all connected to the IP transmission network through a firewall. The application range of the pure IP calling system covers industries of governments and public services, for example, the industries of municipal administration, police, traffic management, energy and the like which are high in information application degree.