When the cell phone rings...
We've all experienced it. A cell phone starts ringing or vibrating. To be clever, it slowly starts getting louder in case the owner didn't hear or feel the initial signal. You see somebody going through their bag looking for the phone that keeps getting louder and louder. Finally they answer and it shuts up.
Yet today's new phones are all featuring accelerometers. This gives them to chance to know we are fumbling for them. Yes, if you're out jogging it won't be able to spot the new level of activity in fetching out the phone, but if you're sitting in a quiet room, and the phone's been still and it starts moving about shortly after starting ringing, the phone can know you're aware of the ring, and start getting quieter. (It might switch back to vibrating or flashing to help you find it.) A ringing phone could also be listening to its microphone, and waiting for you to say commands to it, such as "getting it" to "voicemail" to "speaker" rather than have you hunt for the button. How often has a call gone to voicemail before you could find that button? When you are getting it, the phone could answer (to avoid going to voicemail) and then play a message to the caller about how you are still getting the phone and will be with them shortly. If you can hear the phone, it can hear you.
In fact, phones need to understand more when they are sitting on tables or on our persons. Movement is one way to know this. Temperature is another. Capacitance is a third. Hearing "master's voice" in the microphone reveals something else, like the fact you're in a meeting. Is the conversation half-duplex? You may be on a landline. Many phones know when they are being charged and change their ring behaviour. They can do more.
And here's a second feature for cell phones ringing in the home or office. Let the customer, at that moment, call a special magic number from a prearranged land-line. This number would be put in the speed dial of the land-line, and the land-line would send caller-ID. A call to that number would grab the call going into the cell phone and transfer it to the land line. Ideally the cell phone company would bill these minutes at a lower rate, though I doubt it, since they don't do that for call forwarding set up in advance. This is call grabbing, the way many PBXs can do.
For security there would be a bank of 100 numbers arranged for the purpose. A stranger could not grab your calls unless they knew which of the 100 numbers is the one for you on that landline. Many false attempts would disable the feature, which would be a form of DOS attack but not a very exciting one, as you can just grab the cell phone in that case.
For the user, the UI is simple. You hear your cell phone ringing in the charging station. You pick up a land line, hit a speed dial, and it answers and you are connected.
Of course, the real answer is that phones should never ring at all in most situations.