Really often a telephone system user lifts the receiver off the hook, and then composes the number he/she wants to call (that is a normal behavior also with a Lync deskphone). A standard PBX will connect the user to the number, as soon as it has enough digits to dial a meaningful number. To avoid calling internal numbers if the first digits of the number we are going to call are the same of an internal telephone, we usually have a digit 9 to identify a call that is going out of the company. Lync has a similar feature, in the Dial plan, called External access prefix. This rule will not apply to a call on hook or from a client. For example, if there is an External access prefix 9 defined in a dial plan, any number dialed by the user with 9 as the first number will not be processed by the number normalization rule with internal extension checked in the dial plan.