The company I work for recently received a quote to have a VoIP system installed. The total bill is well over $7,000 and includes a VS-1 Asterisk Server, 2 Analog Voice Cards, hardware based echo canceller, a Polycom conference phone, 5 Linksys 942 phones, a Cisco 871 Router, UPS backup, 24-port switch plus fees for installation and warranty.
To put our needs in context, we currently are a 5 person company (4 full time, 1 part time). 3 of our employees barely use the phone. The other 2 use the phone for probably a combined 2000 minutes a month, maximum. We will probably add another employee or two each year for the next few years, but I don't see our phone needs increasing much.
We are also an IT company. So if setting up a VoIP system is as simple as configuring an Asterisk server and plugging in a few SIP phones, is this not something that we can do ourselves?
My second question is: we have two numbers terminating at our business on the same line. One is a voice number, the other is a fax number. The fax number uses distinctive ring. Can Asterisk handle this? If so, do we only need one analog card, or do we need two? The quote for this VOIP service includes two analog cards (a 4FXO and a 12FXO card).
Thanks!

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It is something you can do for yourself, but depending on exactly what features you need in your phone system and how you want it to be configured, it might be worth your time investing in the initial setup fee (though 7k is pretty steep) to have everything installed for you in a short period of time.
Asterisk can handle fax lines, though how well it can is a matter of how you configure, use it and what hardware you have.
Food for thought:
http://www.voip-info.org/
http://www.asteriskguru.com/
http://www.asterisk.org/

You can always build your own PBX. What's harder to quantify is determining what your time is worth to invest in learning the system and setting it up properly. If you think your time is worth more than $7K to implement the system properly, then let the vendor do it. It might also be possible to purchase the hardware directly, and then get a consultant to come in and sort the other aspects out for you (configuration of the SIP phones, etc)
To answer your other question, Asterisk can handle distinctive ring (also called ring cadence) with only minor issues. We have experienced that sometimes Asterisk doesn't properly detect the ring and shoves the call into the default context which can be annoying. I would recommend using a combination of ring cadence detection, as well as Asterisk's built-in fax detection to route the fax calls to your fax machine.
Good luck.