access-list 101 deny udp any any range 16384 32767 access-list 102 deny udp any any eq 1718 access-list 102 deny udp any any eq 1719 access-list 102 deny tcp any any eq 1720
TCP port 1720 is the port used for the listening of the incoming call. If you block access to this port, you will essentially deny voice call setup request from the IP side.
This will block H.323 call setups, for people using "standard" VoIP and the standard port assignments. It won't block Net2Phone, Netspeak, or any one of a number of proprietary VoIP implementations.
A more drastic approach is to block all UDP traffic on ports >5000, which will kill H.323 and any other RTP-based scheme like MBONE conferencing. It still won't block people who want to get through and are willing to use proprietary schemes.
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