In my area there were actually advertisements on the radio asking people to use the lock function on their cellphone to prevent pocket-calls to 911 a few years back. They claimed it had become a problem.
It leads me to wonder if your comment about capacity (which I assumed was the case as well) does not apply evenly everywhere.
Well, if you don't tell them it's a test call and you pocket dial, I would hope that the 911 call center would assume that you're being held hostage and send police/fire/etc. to you.
So, IMO, that the system isn't meant to handle this type of false alarm isn't reflective of call center capacity but rather fire/police/ambulance capacity (and also annoying everyone involved).
It leads me to wonder if your comment about capacity (which I assumed was the case as well) does not apply evenly everywhere.