Most likely associated with cordless equipment. If the handset or receiver has a 911 feature (one button dialing) , bleedover from a more powerful radio transmitter might cause it, like a gov’t radio in a police car, fire truck, Emergency whatever vehicle. I know of one instance that took weeks to track down the source of the trigger as it was random times of day.
On land lines, a person with a little knowledge of the phone system can make calls “from your house” very easily. Not spoofed, but really over your circuit. Not likely, unless there is a prankster in your area.
Of course, caller-ID spoofing helps. It eliminates the need for the culprit to be in your area or even in your country.
For instance, read about 16-year-old Curtis Gervais:
http://krebsonsecurity.com/2014/05/teen-arrested-for-30-swattings-bomb-threats/