h.323 runs on all IP-Tel solutions however Cisco use their own version of code called SCCP (Skinny) this gives the phones extra functionality. Avaya have their own version, etc.
As far as infrastructure is concerned it depends what you mean. Switching and Routing = Cisco. Servers = almost anything (hardware).
SCCP device work with Cisco’s Callmanager product that can talk to standard H323 devices on the behalf of the SCCP devices. So basically, a SCCP device controlled by Callmanager can talk to any other standard IP Telephony device. Soon Cisco will be introducing SIP support for Callmanager so SCCP devices will soon be able to talk to SIP devices too.