1) For Openfire 3.5.2, if you chose the embedded database, you will more than likely not be able to use the Asterisk-IM plugin as the configuration page has a bug. The only way to fix it is to remove the installation and start over.
2) Ben Perove has a 15-minute screencast on setting up the Asterisk-IM plugin. One thing that he left out is that you need to configure the management user in /etc/asterisk/manager.conf. That's where the username and password come from when you click on "Add Server" under "Asterisk-IM/General Settings". (see Figure 1 below)
3) The "on the phone" function doesn't appear to work well for locally generated inbound calls (e.g., those from my "click-to-call" conference manager). Your mileage will most likely vary.