Your scenario sounds similar to my research.
We have 4 offices though with nearly 10 people in the first 3 offices and 20 in the 4th and 3 satellites from home so a total of 50ish.
Started with Sipura 2000 and voicepulse to make cheap ip lines for outbound calling in the main office hooked up to a couple f outgoing trunks with a twist that the numbers were NY and LA so our offices there have a local number to call and bypass the long distance charges that the telco's like to impose.
From here I am of the opinion that Asterisk is the one and have found a version which is compiled for an apple os x platform (see
www.sunrise-tel.com) which seems to run without error.
The Sunrise site has config applications for putting on extensions and for configuring Freeworld dialup numbers to ring on the mac switchboard and get passed around.
I have Asterisk now working at home, and I have purchased the new "mac-mini" for $599 (faster processor) and loaded Asterisk on that for the office application once I master the home config and get comfortable.
Next step will be cleaning up the quality of service issues at the office so calls dont get killed when internet bandwidth gets heavy.
Will decide on a desktop VOIP phone in the next couple of mths and have just ordered one of those Sipura phones to try.
The plan will be to move our incoming lines across to something like voicepulse connect - they have many area codes at $8 a month with no inward charges so seems perfect for us. Calling out costs 2.9c/min and I am sure there are others cheaper so mix & match may work.
When I begin the migration toward asterisk and toss the old fashioned switchboards, I will probably augment the traditional "backup" pots lines in each location by plugging them into a Sipura 3000 that will give us options as no one has written os x drivers for the digium PCI card yet and I don't want to be the guinea pig !
Additionally, because of the flaky nature of PC's and Operating systems, I intend buying a 2nd mac mini to backup the first and will possibly place that in one of the other offices to backup the other.
All that will do a lot for us and at not much cost and without the need to be a Linux rocket scientist and we will have some reasonable chance of controlling it and document.
There is also an initiative to develop a GUI software interface for asterisk so you don't have to get deep and dirty @ the command line to configure things - hopefully a quick port to mac will happen.
Now here's the real cherry - we have just setup blackberrys and interfaced those to our switchboard via a cellular gateway. With everybody able to call each other for free through the cellular vendor because we are a "group" - like a family plan, we also have those "gateway phones" pegged to our switchboard - once these numbers are called for free, the users can dial back out over voip lines and bypass the cellular costs almost completely.
We also have internet on those blackberrys, and a really nice touch would be a SIP or IAX client for Blackberry to make VOIP calls direct
RIM the blackberry manufacturer aren't stupid though and don't want to kill their business with the cell vendors too soon, so they have just released a "campus" blackberry that works over wi-fi not cellular. its only a matter of time though until the 2 come together.
Hope this insite helps.
Shayne