If my phone started ringing and wouldn't stop, it would go something like this:
Answer phone, "hello?"
Try the end call button
Try the button on the receiver
Remove the battery from the phone
Unplug the receiver from the power supply
Unplug the receiver from the phone line
Start to get suspicious
Dismantle phone and isolate ringing part
Take a video of it ringing without any apparent power supply
Get better microphones/cameras and repeat
Show to a few other people to confirm it isn't just latent schizophrenia or something