Not just because it was a bank. It's CPS' policy that any and every time a phone calls 911, that extensive due diligence be done to attempt to find the phone and figure out what's going on. I've only not been able to 'find' a phone once, and that's because the person on it utterly refused to stop moving around the city and talk to police. After an hour and a half, it was decided that they were clearly not in any immediate danger, and I was allowed to stop trying to make contact.
In my experience, it's not uncommon to have around 50-150 of these a day. I'm not sure what they'd average out to over the year, but weekday 'business hours' are obviously the worst, while night shift (or rather, past 8pm) almost never sees them.
Businesses are the worst for it, as you noted most have to dial '9' to get out. What makes it even trickier is that most businesses will dial 9 for an outside line, then 1 for long distance, then take a couple moments to look over the number and...by that time, your phone may assume you are trying to dial 911, fill the extra 1 in for you, and then you are talking to us. I don't know what the criteria is for a phone to fill in the extra 1, but a lot of them do, as the number of people I hear who say some variation on "I was dialing long distance and just paused to make sure I had the right numbers..." is very high. Because of this, businesses are slowly starting to switch to 8 for an outside line. However, I don't know what that's doing to Healthlink's false call rate, as they are 811.
Next, we see a lot of folks who use those Long Distance calling cards doing much the same thing as the businesses above. Those are the two biggest situations that we get 911 misdials from.
It is hard (for me, at any rate, given the access to the numbers I have) to pin down an exact number, as the ones I can look at class a 911 misdial and a 911 hangup as the same thing. So I can't get a solid number on either.
In my personal experience, I'll deal with 1-15 a shift, depending on the shift.