Don't leave me hanging on the telephone

Why is it impossible, with a landline phone, to hang up and get a clear line when someone's called you with a mobile? Boy didn't believe me, but I showed him that, if the caller with the mobile doesn't cancel the call, the landline phone can't get a clear line again, no matter how many times you hang up the receiver. It's infuriating when it happens to your home phone (when your Beloved has the mobile in his pocket and all you can hear is the sound of his rhythmic stride), but it happened at work and I had to spend a full 20 minutes shouting and whistling and desperately trying to attract the attention of the mobile owner. Silly twit had accidentally speed-dialled the vet's number without realising. I hope nobody wanted to contact the vet urgently.