Imagine you are sitting with a co-worker and you hear your phone ringing. Instead of running back to your desk, simply dial a directed call pickup.
Your extension is normally a 4 digit number (e.g. 3000).
To pick up a call you have to consider which line the calls will ring on. Normally calls will ring on your line 1.
To answer a call on extension 3000 on line 1, you would dial:
or more generally
** + Extension + Line + #
The # forces the phone to dial immediately and may not be required on some phones or if you are trying to program a speed dial.