The console is usually set up to answer incoming calls on a 'first-come, first-served' basis.
When you press the Answer (Enter) or 
or the Answer/Release button on your headset
with multiple calls waiting, the call that has been waiting longest
will be answered first. Alternatively, you can assign priorities
to calls based on their destination. For example, you might give higher
priority to calls to your company's main listed directory number
(LDN) than to Dial 0 calls (i.e., internal calls to the console).
See Options (Answer Priority) to
configure these options.
Note: The Answer key has a hierarchy
when the console is set up to answer calls on a "first-come, first-served"
basis: recalls are answered first, followed by other calls either
in order of call-answering priority, or in order of arrival. If
more than one call is waiting at a particular softkey, those calls
will be answered on a “first-come, first-served basis”.