There are two type of calls which your phone will automatically answer the call so that you automatically hear the caller. This will only happen if your phone is currently idle, ie. not on another call.
A call connected automatically after three beeps is a direct voice call to your extension only. The call is connected and answered automatically. You can speak to the caller without taking any further action.
A call connected automatically after a single beep is a page call a group of which your phone is a member. You can hear the caller but they cannot hear you.
To answer the call, turning it into a normal call between just you and the caller, press Feature 3 or press the Answer soft key.