SIP for Telephones provides a framework for the integration of legacy telephony signalling into SIP messages. It provides two key characteristics, namely ‘encapsulation’ and ‘translation’. The SS7 ISUP messages arriving at a SIP-ISUP gateway are ‘encapsulated’ within SIP; this makes sure the information necessary for services is not discarded in the SIP request. However, routing decisions for SIP requests are made at proxy servers which cannot be expected to understand ISUP messages. To overcome this, some of the critical information is translated from an ISUP message into the corresponding SIP headers, allowing the SIP request to be routed.