A few months ago when staying in a Hilton somewhere (Leeds, UK I think?) I received a phone call from the front desk whilst I was ironing a shirt. Turns out the steam from the iron had triggered a fire 'pre-alarm', and they were calling to confirm that there wasn't a fire and to let me know that the manager was on his way to confirm that and would require immediate access to my room. If they didn't clear the alarm within a certain time period it would have triggered the hotel fire alarm.

So yes, there certainly are valid reasons for leaving the phone connected...

By contrast, I stayed at a Permier Inn near Manchester, UK recently where the rooms didn't have any phones, but you could call the front desk from your mobile phone for only ~10p/minute!