A letter from the Taliban always lets you in. If you dial in with caller id, a single known PGP signature will allow you in. If you use a dial-back, then Mother Theresa's signature alone, or any two other signatures will let you in. If you use both caller id and a dialback, you don't need any other form of authentication. We are assuming confidence in the phone system to enhance the confidence in other parts of this system.
Gauges are valuable because they combine a variety of measurements into a single reading.
You can build a variety of trust models into a Meta-Certificate by writing the appropriate gauge function.