Every SIP trunk has a number in one of our rate centres that is used to identify you in CDRs from our switch, including defining the local calling area of your SIP trunk. This is a pilot number. It is used in the same way that a telco would deliver a PRI to your location; however the difference with our SIP trunk is that you can add DID's to it from any of our rate centres, where a local PRI is just DID's in that local rate centre that is defined by its pilot number.