Comment Use the tools you already purchased first (Score 4, Informative) 224
If you've already purchased and using (albeit only barely) Microsoft SBS, take advantage of Exchange before you spend any more money on a new system, otherwise you're just wasting money. Exchange works quite well, obvious straight-forward connectivity with the Outlook client. Administering Exchange isn't the end of the world, and is actually quite easy in an SBS environment. I would suggest setting up an alternate internal smart-host (smart-relay) so that you don't have to expose the Exchange server directly to the internet. Courier MTA works VERY well (and is the exact setup we have internet->courier->exchange).
Setting up a Jabber IM server internally is easy as well, otherwise use Google Apps and have your email domain hosted there and just use Google Talk with the various AV plugins.
Setting up Switchvox (Asterisk) is a purchase, but I 2nd the comment by others to find you a local phone service retailer and let them deal with phone integration. If you do decide on a hosted solution for email and voice (voip) then make sure you don't skimp on the internet connectivity. I worked at a place previously convinced VOIP was the way to go, but management would cringe every time you talked about capacity of the external connection and the need to upgrade.
Just my 2cents...