uthenticate when making outgoing calls. Ten different SIP providers may have 10 different methods for connec- tivity. While the underlying principles and protocols are the same, don't fall into the trap of thinking that what you see here will work as-is on any SIP trunk. By following this example, though, you should be able to determine what you might need to change to make your system work with your provider. There are a few sections of the SIP configuration that we need to address. First is the voice service voip section: voice service voip allow-connections h323 to h323 allow-connections h323 to sip allow-connections sip to h323 allow-connections sip to sip no supplementary-service sip moved-temporarily no supplementary-service sip refer If you're building an actual system from this chapter, now that we're working on SIP, make sure to shut down your voice port and POTS dial peers SO that they don't interfere with what we're doing here. The first few commands should be pretty obvious. The last two commands disable the SIP 302 moved temporarily message to make call forwarding work better on a CME system. The last command addresses the CME, sending a SIP REFER message when calls are transferred on CME. Since most SIP providers don't support the SIP REFER message, this forces the CME to hairpin the call instead. We need to do three things to make our SIP trunks work at a basic level: tell CME where the SIP proxy is located, authenticate outbound calls, and register our numbers SO that the SIP provider knows where to send calls destined for us. We perform all of these actions within the sip-ua configuration mode. First we'll define the SIP proxy: R1-PBX(config)#sip-ua R1-PBX(config-sip-ua)#registrar dns:sbc.sip-provider.com expires 3600 R1-PBX(config-sip-ua)#sip-server dns:sbc.sip-provider.com Chapter 30: VolP Notice that the SIP servers are referenced as DNS hostnames. You can reference them as IPv4, IPv6, or DNS names. If you enter an incorrectly formed address, you will be treated to this most excellent of error