An architecture for a communications node and a method of implementing control functions in a Session Initiation Protocol (SIP) telecommunications network. The node performs a plurality of call-control functions in a single physical node. A plurality of application-level logic blocks corresponding to the plurality of call-control functions are interfaced with a common engine module which is implemented on top of a common operating system and physical platform. The engine module includes SIP behavior functions and SIP stack functions, selected ones of which are operable to perform different call-control functions when interfaced with different application-level logic blocks. Mapping tables within the engine module identify groups of functions that, together with selected logic blocks, perform defined call-control functions. The open architecture allows additional functional logic blocks to be interfaced with the engine module to implement additional call-control functions.