My friend L J Rich discovered that in the 5th Carriage, Westbound Central Line at Mile End, she could make a phone call.
I bet there must be other parts of the Tube like this, which it would be really handy to know about if you needed to make a lightning quick call or check emails, or send a text. (I know there will be a number of naysayers for an idea like this, but when, like me you travel half your journey overground on the Tube, you'd be surprised how few people actually use their mobiles with inane chatter, simply cos it's too noisy).
There's certainly a bit underground on the District Line around Sloane Square where you can call.
Perhaps it's possible to have a map of mobile phone hotspots. I bet there's probably some correlation between the depth of the station, the proximity to masts and your network provider. Anyone think it's worth doing or have any theories? Do you know any secret hot spots on the Tube where you can call underground?