Most people have cordless phones. How do they work in a blackout? A good proportion of people don't use a landline phone. How do they work in a blackout?
The answer is that this would have seemed like a problem back 20 years ago - remember when rabbit cordless phones came out? The base station had a 9V battery backup. That was the first digital cordless phone on sale in the UK, and people were probably genuinely worried about such things. These days... I think not so many people care so much.
Biting the hand that feeds IT
