If you call 911 accidentally and then hang up right away the 911 operator will try to call you back.
If they cannot make contact with you then they will try to locate you by looking up GPS of the cellphone or if from a landline they can locate the address where the landline is.
Then a police officer will show up at your address to see if everything is okay.
If it's an accident then when the police show up just talk to them and then they will go away.
It has happened to me before where I had butt dialed 911 and I hadn't realized it but then about 20 minutes later the police were at my house.
I explained to them and they just checked if everything was okay and they were on their way.
The police have to show up on a 911 hang up call even if it's not an emergency because they don't know that.
Some people are forced to hang up on 911 by the criminal so they have to check it out.
Or if you called 911 and then hung up and it happened to be that you passed out or was having a heart attack etc they have to check the call out just to be sure.