Help designing best fit

I work for a small neurology office and have been asked to look into phone system options/alternatives to break this office away from a larger group with which it shares a traditional phone system. I was wondering what it would take to run the following phone system profile using Asterisk, and what would be recommended hardware and software (plain asterisk, asterisknow, or other derivatives).

16-20 phones with individual extensions that can be grouped (transfer to a certain hold queue would ring a handful of phones)
maybe 5 actual phone numbers
10’ish voicemail
primary line goes to simple menu system (maybe 2-3 layers deep at most)
5-6 simultaneous calls/connections with the outside world.
Fax line - in and out - capabilities

What hardware layout (single beefy machine, a few small ones, etc) and what types of phones (soft phones preferably avoided) would work best for this? I know asterisk can run on almost anything, I’m just hoping to find a good balance of performance, cost, and function and this is all new to me so hoping to get some feedback to turn a big foggy picture of ‘what can be done’ into something a little clearer.


asterisk :
elastix could be a good choice. we are also using it in our office the ui looks intuitive and it already provides a number of features (eg voicemail, conference, extension group, etc.) for office use.

extension phones :
it could be analogue or ip phones. if you use ip phone, beware that you might need extra power socket or lan port for each user. it is worthwhile to consider PoE ip phone with 2nd LAN port to connect PC. an example is gxp285.

for analgoue phones, you need fxs gateway.

fax capability :

use an ATA device to connect your fax machine to asterisk.

ivr for primary line :

you can build a ivr using the elastix ui.

pstn phone numbers :

you can obtain pstn from telco or some ITSP. the telco will provide you co lines (1 line=1 call) or T1 line (1xT1 = 23 calls). in any case, you need to install a tdm card (eg from digium) in your asterisk server. the tdm card has t1 port or fxo ports where you plug in the telco lines.

in the case of ITSP, they provide telephone numbers in the form of internet sip trunk and in/out calls are now ip-based. you don’t need to purchase extra t1/fxo card. however, a stable internet bandwidth is required.

alternative :
the market has some pbx appliance which combines the above within a compact device to ease your installation.

Thanks, that covers some of the things I’m looking for - but what about things like CPU, memory, disk space and etc? Should this be it’s own machine, or should I place it in a VM on the office’s medical records server?

VoIP ideally requires scheduling latencies of less than 20ms and latencies of 200ms will cause choppy audio. If the loading on the VM host will not allow this, you should use a real machine.

Memory and disk space needs? I’m going to just assume a modern era dual core (at least) or quad core processor. What about the connection to the phone network itself - can it all be done via TCP/IP and a solid net connection, or does there need to be at least one actual land-line for the fax? What sort of bandwidth would be needed if using the internet to support that many simultaneous calls at once?