Bring it on!
I don't see any downsides to this at all.
1) Typical communication has already largely moved from voice based to text and image based anyway. It's not like buses are no-go areas because dozens of people are having a voice call, why would the Tube be any different?
2) Sixty percent of the Tube is above ground and even then the Tube is not anything resembling a call centre. Why would people suddenly want to call when they are at an underground section?
3) The ambient noise levels are too high for any conversation. If the Tube would be a new product it would not be allowed to operate due to the dangerously high noise levels, the only reason it's legal to travel on some lines without ear protectors is because of its legacy. You'll notice that some drivers do wear ear protection.
4) People worried about terrorists setting off devices remotely haven't noticed that in 2005 "7/7 bombings" they set off devices by hand and that explosive devices on timers have existed for easily a century.
5) It's great to be able to tell your boss, partner, date etc. that you've been stuck in some tunnel without movement for 45 minutes now and are going to be late.