I replied privately in detail on this one, but for the benefit of others, I connect into my office's PBX (Asterisk, open-source). It's a pre-2010 install of Asterisk, so I don't remember which version, but it's old, running on even older hardware. (Anyone want some 36GB SCSI drives?). I'd recommend people try something like IncrediblePBX or FreePBX today. Our office buys our SIP termination/origination wholesale, so actual service doesn't cost much.