Our policy is dependent on the type of hang up.
If we hear something suspicious we'll send law enforcement and other agencies we feel are necessary. Other emergency crews will stage away until the law enforcement clears the scene. If it's a cell phone, it also depends on whether they stay on the phone long enough for us to get a decent idea of where they are calling from. It's surprising how long it takes and how low the precision is in the tracking technologies used in 911 centers.
We always attempt callbacks on hangup calls, but a lot of the time we get persons calling from expired prepaid phones which will only dial 911 and not receive calls. The majority of our 911 hang up calls come from children who have been given old phones, taken their parents, or given expired prepaid phones... Parents don't realize that any cell phone can dial 911 even if it doesn't have a service plan.