How to notice a phone is ringing when left in a bag?
I could think off two possible alerting devices like:


A Bluetooth enabled watch - which when paired with the phone can display incoming call alerts and can even vibrate. Being a watch on our hand its vibrations can't go unnoticed. Casio has one; Amazon has few bracelets; Sony's LiveView are few options.
To wear a Bluetooth headset always so that the ring can be heard. If that looks like working for FBI :) there are jewel kind of BT devices like this one.