An implicit precedence level can be assigned to an extension and is used automatically when a call is made. A use for implicit precedence might be to assign a high precedence to an emergency phone located in a public area. In either case, the level of precedence is established before the call is connected and is always determined based on the calling party. During the connection process, if the destination is busy, the PBX compares the precedence levels of the established call and the incoming call.

If the incoming call has higher precedence than the established call, the called party may be notified of the higher-precedence call with a display message or voice prompt. If the user does not disconnect from the current call within a set time-out period, the call is dropped automatically, and the call with the higher precedence is then connected to the called party. For cases in which the incoming call does not have a higher precedence, it is handled normally (busy signal, voicemail, auto forwarding, etc.)

Pre-emptive calling is a feature that can provide valuable internal services for enterprise-level PBXs. For instance, a company with remote sites can set up a hotline for each site to the system administrator, so that if something goes wrong the remote site can get immediate help. Other applications include security red phones, special links to business partners, and emergency service notification (police, fire, 911).

Precedence can also be used to seize outgoing or networking trunks if needed in emergency situations. In the case where all of the trunks are in use and a high-precedence call is initiated, one of the calls at a lower precedence level will automatically be disconnected to free the required trunk resource.