These first couple questions concentrate on your business requirements.  Then we'll jump more into the technology requirements, your office's infrastructure, and finally your personal preferences.

How important are the calls to your business?

I'm going to break this question down into two parts:

1) How important is call completion to your business? and
2) Are your calls highly perishable?

When I say call completion, I mean, how important is it for an incoming call to be answered by a live human being for most, if not all, of your calls?  Let's say you're in the office kitchen, prepping a bagel, and your desk phone begins ringing.  Is it of utmost importance that you answer it?  Or can it go to voicemail and you'll deal with it later?

If you answer, "I need to answer that," then call completion is very important to you.  There are a number of ways to accomplish call completion in the above scenario, and I don't want to hit on all of them here, because I'll end up confusing you.

But here are two possible solutions:

A) Have your phone system automatically page over all phones, including the phone in your office kitchen, "Call for John."
B) Wear a headset that will beep in your ear when your desk phone is ringing; then you can pick up the call through your Bluetooth earpiece.

Are your calls highly perishable?  A classic example of this would be a real-estate company.  You want to make sure that someone is available to pick up from any phone on every incoming call.  If you miss that call, that antsy, prospective home buyer is going to call some other agency.  This is a case where calls are highly perishable.

In contrast, a huge company like Microsoft has completely non-perishable calls.  If you need support for Windows 7, you'll call their support line and sit on hold for 2 hours.  Why?  Well, who else are you going to call?  Apple?

I hope by now you understand why this is an important question, and hopefully you have an answer.

So you begin wondering, how does this apply to the purchase I'm about to make?

Don't stress too much on the end result quite yet.  Just make a note to yourself of your answer.  Some phone systems can better handle call completion.  When we dive deeper into desirable phone system features, you'll find that these preliminary questions play a big role.