What you want to do is fairly easy with Asterisk, and I have successfully used Asterisk with a P3 in a scenario similar to yours. You didn’t mention if you had high speed internet access available, so I’ll give you 2 scenarios you can mix and match.
The bare bones system you can do for very little additional cost and assumes you are using 1 plain analog incoming line:
Depending on your finances, get an X100P card from Ebay for about $15-20. These work well if you have a good phone line, but can be problematic if your line is noisy but they’re so cheap it’s worth a shot. If you want to be sure it works first try get a TDM400P card with 1 FXO daughter card (it will be called a TDM01b and will cost about $150) Either one of these will handle the interface to the outside phone line. For the bare bones install, you only need 1 phone on the system which can be either a hardware IP phone or an ATA (Analog Terminal Adapter) that allows you to plug in a regular phone. For flexibility and easy setup I would reccomend a Linksys PAP2 ATA. For about $60 you get two analog phone jacks that allow you to have 2 internal extensions in Asterisk.
You will need to set up 1 extension per person in Asterisk to allow for individual voice mails, but you don’t have to have a physical phone connected to each one, as you can access individual voice mail from any extension by authenticating with the system.
Also you’ll need to set up an IVR (Press 1 for Joe, 2 for Mike etc.) to allow callers to leave voice mail for the correct person, something Asterisk can do without a strain.
So, the barebones system will have 1 incoming analog line connected to Asterisk and 1 or 2 physical phones connected inside.
If you have high speed internet available and can spend a little more you can do some way cool stuff You can sign up with an Asterisk friendly VoIP provider such as Telasip or Broadvoice with a Bring Your Own Device plan and either use it in conjunction with your analog line or arrange for 2 “lines” with the VoIP provider. This will allow you to set up an IVR with options such as automatically forwarding the call to specific cell phones by user, i.e. when a caller presses 1 for Joe, Asterisk will forward the call to Joe’s cell phone.
If you can wipe out the hard drive on your machine, I would reccomend Asterisk@Home. (asteriskathome.sourceforge.net) It’s a downloadable ISO that installs Linux, Asterisk and a whole slew of other goodies and makes things pretty easy to configure.
One of the things you’ll discover about Asterisk is that it’s not “can I do this with Asterisk” because usually you can, it’s “how do I do this with Asterisk”. It’s an amazing piece of software!
www.nerdvittles.com is a treasure trove of how-to’s based on asterisk@home. Read his “soup to nuts” guide to asterisk@home and you’ll be well on your way.
Also, here are some reputable VoIP hardware suppliers that either myself or people I know have dealt with: