I keep my phone in vibrate or silent mode most of the time. But when my wife calls, if I happen not to hear/feel the call, I'm in big trouble!
Apparently some devices including my HTC Desire do not allow Tasker to change the silent/vibrate mode status once an incoming call sequence has started.
So the trick is to play your ringtone file using the media player when that special someone calls, and turn off the media player when the call is answered or declined.
The media play should be triggered whenever the ringer volume is lower than some threshold. This covers both silent and vibrate mode.
Turning off the media player on call decline can be done in the exit task, but you also need an extra profile triggered by the Phone Offhook event for when the call is answered.
Profile "Ring when XX calls"
State: Incoming Call
Caller = XX (push the magnifying glass icon to search for a contact to insert)
First task "Play Ringtone"
1. If %VOLR < 5
2. Media Volume = 14
3. Music Play, File = media/audio/ringtones/myringtone.mp3
4. End If
Second task "Music Stop" as Exit Task
1. Music Stop
Profile "Turn Off Music On Offhook"
Event: Phone Offhook
First task "Music Stop"
Thanks to the tips in the mailing list archive for the basic idea.