Use this if you have newer software and a PRI in stead of DISA. Works a lot better.
Call Recognition Service
Call Recognition Service (CRS) checks the calling line ID of the incoming call to determine whether the caller is an external hot desk user (EHDU) and whether the trunk is "trusted" or "non-trusted." If the caller is an EHDU on a trusted trunk, he or she is automatically logged in. The digits dialed by the EHDU are then processed and the call routed to the intended station or other destination. If the digits terminate on the Hot Desking Access Number, then the user is provided dial tone and may continuing dialing.
A non-trusted EHDU must enter his or her PIN to log in. Without CRS, the EHDU would have to dial the Hot Desking Access number followed by their hot desk user directory number and User PIN to log in (unless they have "Permanent Login" enabled in their Class of Service).
CRS also provides integration with the Closed User Group (CUG) services (also known as Force-to-PBX) offered by some mobile phone carriers. With CUG, all calls to, or from, a mobile subscriber are routed to a PBX over a dedicated trunk for processing. When used in conjunction with EHDU, the PBX can recognize calls made from mobile devices as originating from an EHDU by matching the PSTN number of the device to its EHDU directory number. To recognize that a call is being made to the mobile phone, the PBX matches the called party number (i.e., the PSTN number of the mobile phone) with the EHDU. Once recognized, the PBX substitutes the PSTN number with the EHDU directory number and routes the call to the EHDU. And since the PBX is always in the call path, it can maintain device state (busy, idle, call forwarding active, etc.) and offer mid-call features through DTMF detection.
I'd tell you a UDP joke but I'm afraid you won't get it. TCP jokes are the best because you always get them.