I use Asterisk, managed via FreePBX. It works pretty well if you don't have that many users. I use a SIP trunking service with a DID. Siphon (SIP client) for my

I use Asterisk, managed via FreePBX. It works pretty well if you don't have that many users. I use a SIP trunking service with a DID. Siphon (SIP client) for my iPhone, Telephone on OS X, and also forwarding to my normal cell. I've heard really good things about Shoretel, but I know its very expensive.