As one enters a WebEx meeting, the WebEx client communicates with the conferencing server to establish a reliable and secure connection. In the process of establishing the connection, the WebEx client first attempts to connect using TCP (port 1270). If this port is blocked by a firewall, the WebEx client will then tunnel WebEx communications using HTTP (port 80). (If a customer elects to purchase and use SSL the traffic is carried over port 443.) By establishing communication between the thin client and the WebEx servers in this manner, customers' firewalls do not have to be reconfigured to enable online meetings, thus complying with corporate security policies that guard against the opening of firewall ports.
So, I think that port 3389 may be the wrong port to block, but thanks for the suggestion.