Unfortunatlly everything goes quite well with Cisco if the entire network is only Cisco. We've managed by mistake to convince in several situations Cisco phones (7940) to reboot after a well formatted SIP packet(RFC 3261 compliant) that wasn't in the way Cisco thinks SIP should be.
I think that free/open software is starting to be backed up by companies that are able to provide the technical support for any kind of issues. A few companies which do that are: Null Team which supports Yate, Digium which supports Asterisk.