Thanks P Dev
The laptop is capable of Skype but I've never downloaded it. I had been on the internet. I returned to the desktop afterwards. I was long enough away from the machine for it to have gone into the black screen. There are three mobile phones in the property. None were switched on.. There is no other device near the laptop.
The girl's voice which said "hello" had a surprised inflexion as though the ringing on ( presumably ) her phone was unexpected.
Baffling !