A method for implementing communication services includes: after receiving a call request, determining whether a user subscribes to an IP Centrex service and an intelligent network (IN) service according to the obtained subscription information of the user; if the user subscribes to the IP Centrex service and IN service, checking with an application server (AS) whether the call request is for an intra-group call; if the call request is for an intra-group call, performing corresponding service processing according to the checking result to implement discount charging on the intra-group call. By using the method of the present invention, if the user subscribes to the IP Centrex service and the IN service, no conflict occurs between the IP Centrex service and the IN service, and discount charging is implemented on the intra-group call. A network device and a mobile communication system are also provided.