Yeah, most carriers have an agreement with the WiFi service available in stations, but it's easier to get on the network with some carriers than others. When I was on EE, I could just authenticate through the Virgin Media portal, but with O2 I have to connect to 'WiFi Extra' and use my SIM as authentication.
Anyway, if you're connected to the WiFi and you have WiFi calling switched on, you should be able to make calls from underground. And it will show you as having some signal, though really it's just WiFi if that makes sense.