A method of processing session initiation protocol (SIP) call flows, the method comprising, at a back-to-back user agent (B2BUA) maintaining a plurality of predetermined call flow behaviours, each call flow behaviour in the plurality being self-contained and independent from other call flow behaviours in the plurality, each call flow behaviour in the plurality being applicable at one or more given call flow stages to provide one or more given actions for addressing a given call flow scenario, in response to occurrence of a given trigger event in a given SIP call flow, selecting a call flow behaviour from the plurality of call flow behaviours, and causing execution of one or more actions associated with the selected call flow behaviour to address a call flow scenario in relation to the given SIP call flow.