Free (beer | speech) SIP PBX servers?

Hello

Since the free beer Axon PBX I'm using to build an SIP PBX server is
missing a feature that I need (namely, include a user in a group
extension although he's registered on a remote commercial VoIP
provider so as to be notified of calls on his GSM cellphone), I need
to look at alternatives.

Besides the following, do you know of free beer/speech softwares to
build a PBX server?

- Asterisk ..
Telecom, Cabling and VoIP continues...
Tags: Telecom cabling Networking Broadband