"we asked about VoIP but didn't get a clear answer. "
The shape of things to come?+
Transport for London is tendering for bidders to open Wi-Fi hotspots at underground stations, and possibly even at bus stations and stops. The decision follows a trial with BT OpenZone at Charing Cross tube station in central London. Transport for London said the tender was for up to 120 tube stations, out of a total of 260 …
in the absence of VOIP, the only way you'll be hearing your neighbour's conversations is if you lean over their shoulder and read them yourself. And then you'll get both sides. Or if someone in the carriage talks to someone else in the carriage. Never been on the Underground - is that verboten?
I work in Paris (the one in France) and pretty much the whole Metro network and RER network have mobile phone access in the stations and in most of the tunnels.
I don't think the French authorities see it as a security risk. I have no idea who paid for it, but I imagine that the mobile companies make a tidy bob or two.
London Underground is older than the Bible* so all additions are extensively retro-fitted. Other countries (and cities), such as Germany (and Newcastle) have more modern affairs that were built with a bit of expansion-planning in mind.
Mind you - tech is so small nowadays it surely can't take up THAT much room!
*Nearly..