SipXecs is really great. Nortel actually bought PingTel and called it SCS (terrible name). For a moment it looked like there was some hope that the industry was going to see the light, but after the acquisition Avaya promptly killed it off.

Fortunately, because it was open source, we now have SipXecs. Last I worked with it (5 years ago), I'd be comfortable implementing it for a client with less than 1500 users and no complex contact centre. I imagine it is much more capable now. I still recommend it.