If you called the number the PBX from your cell phone, waiting for the prompts of IVR, simply dial the extension number. Even if the tone signals thus you will not hear the lines they're given to still be. If the person you wish to reach is on the phone, soon you will be able to talk to him. If it isn't there, long on the line don't stay, because the billing of the call starts from the moment of answer of the system.
When calling from a landline all depends on what kind of PBX you have. If it supports tone dialing, then your home machine, most likely, long been transferred to the appropriate mode with the appropriate switch, and dial an extension number immediately after the invitation. If not, typing the number of the PBX in a pulsed mode, wait for the prompt of the system, and then press the star (it will switch your phone into tone mode) and then the extension number. After you talk and hang up, the device automatically switches into pulse mode.
Telephones with disk roomby habitaculum as early models button in tone mode can not work. If you often need to call on a PBX, and the phone you have does not support this, buy a special device - a beeper. It consists of keyboard, electronic synthesizer, DTMF, and speaker, and is powered by batteries. It is enough to bring a speaker of beeper to the microphone handset and dial the appropriate digits and the PBX detects the extension number.
If the telephone does not support tone dialing and the beeper there, and also if the extension number of the desired employee, unknown to you, after the invitation of the autoinformer do nothing, and after half a minute you'll meet the Secretary of the organization that you are calling. Call him the name of the person you want to talk to and he will switch you to the appropriate extension number manually.