The 3rd Generation Partnership Project (3GPP) uses the IETF Session Initiation Protocol (SIP) as a signalling protocol in the IP Multimedia Subsystem for 3rd generation UMTS networks. Signalling Messages sent using the SIP protocol pass through intermediate SIP nodes in such a way that the message size grows as a result of additional data being added to the message. Modelling of network flows in this case requires careful attention. A simple flow model is presented to get an appreciation for the size of the expected signalling flows. This is particularly important for developing dimensioning models for SIP in 3GPP networks and for the further investigation of Quality of Service (QoS) mechanisms to provide resources needed for signalling. A methodology is presented which defines the minimum requirements for the bit error rate on links used by signalling traffic.