Whilst it's a noble thing to want to go open source and take it all on yourself, you can save yourself a major headache by going SaaS instead. I originally setup a XenServer running VMs for Endian Firewall for routing, Zimbra for groupware and Asterisk / Trixbox for the phones. It was, to say the least, a pain in the ass to support, and there's no place to hide when you're the sole admin for a system you setup yourself.
About 8 months ago I got sick of the distraction it was causing from my main role, and now we're running Google Apps for Organisations, a hardware router and a Sipgate Business account for fully hosted VoIP. We use Cisco SPA-921 SIP phones, which are about £55 (roughly $90), and the whole setup causes zero headaches. Our old setup appealed to us on principle (we're all software engineers!) but that was all it was - in the end, it cost us time and thus money.
It helps that we've got a great, rock-solid broadband net connection (check out Fluidata if you're in the UK and looking for good business connections!), but the combination of virtually zero critical equipment on-site and hosted services is such a good economical decision.
Yes, I know I'm preaching to an OSS-biased community here - OSS stacks can be great, and we probably will end up going with some combination of in-house and SaaS in the future, but only when we have dedicated people to run it. In the meantime, get the technology out of the way so you can concentrate on whatever it is your business does!