what is a "PRI"? Need understanding

We have 6 trunks coming into our company.  
Our problem is that we are getting too many busy signals when outside calls try to get in.
We have 80 DID's and about 80 users.

If we got a PRI, what does that mean, exactly?  
is a PRI considered additional "trunks?"
If we had 6 trunks plus a PRI, does that mean more than 6 simultaneous calls?
Should we be looking at getting more trunks *and* a PRI, or is just getting a PRI good enough?
someone please just clarify for a telecom newb.  Thank you!
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With 6 lines and 80 or so users I can imagine you do receive busy signals quite often.  That is not a lot of trunk lines for that many users.  Especially if your users use the phone regularly.

A PRI would allow for 23 simultaneous phone calls.  It is actually a 24 channel circuit but one of those channels is used for signalling and control.  A PRI itself is good enough.  The only reason you might want to retain 1 of your old analog trunks is for redundancy in case the PRI circuit is down.

A PRI is a completely different scenario than what you currently have.  A PRI is a 2 or 4 wire digital circuit.  Long story short it allows for 23 phone lines to come across the single 2 or 4 wire PRI circuit.  In order to do PRI you do have to have a T1 or PRI card in your Mitel.  The reason people use PRI's is that when you get above a certain number of trunks it is more cost effective to do a PRI and get 23 lines.  For example - if analog trunks cost you $50 each and a PRI costs $600.  It is more cost effective to go with a PRI and get 23 trunks when you get above 12 or so lines.  The other reasons to go with a PRI are that it is a digital circuit, sounds better, offers enhanced Caller ID capability etc.

What I would recommend for you is to talk with your phone company and see what their pricing is for analog lines versus a PRI circuit.  You would also have to verify that your PBX has the capability and has a digital line card you can use.  Even if it does not you may want to investigate adding one.  It may cost you know but the payback could be pretty quick.  Especially if you really need the additional lines.

I would still recommend viewing the call detail reports as well.  Just to get a true understanding of how many lines you are using.  

Here are some links for you to learn more:

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