Also known as coordinated dialing plan.An optional PBX feature for large, complex multisite PBX networks, UDP supports simplified station-to-station dialing, allowing multiple PBXs to share a single numbering plan wherein each station has a unique number comprising four or five digits. The end user need only dial those digits to be connected to another user, regardless of where the two are located. If the two are served by the same PBX, the call connection process is simple. If the two are served by separate PBXs, the call will be sent over a private facility such as a tie trunk if one is available. If the call is sent over a central office (CO) trunk for connection through the PSTN, the originating PBX will automatically insert the necessary country code, area code and central office prefix, or otherwise modify the dialed digits as necessary to, for example, direct the call to the DID number of the called party.