I'll give you two options:

If your calendar event was "meeting" you could make a rule that checks for meeting with the "NOT" option enabled, and sets it to ringing.

Since I let you set the order of rules and it processes them top to bottom (I think) you can just put a rule at the top that doesn't check anything and sets it to ringing, and just make sure you have rules below it to set to quiet when you want it to.

Let me know if this doesn't work for you...