There is no security or regulatory prohibition to use cell phones inside the Eurostar. Of course, phones don’t work at all when inside the Channel Tunnel. During the rest of the trip the train goes so fast that the phone is not able to maintain a long connection to the network so it is very common to lose a voice connection. SMS messaging works fine. Data connections for Blackberrys also work, but due to the intermitting connection it is not very much real time.
Given the relatively low noise level on the train, it is certainly not recommended to call when seated. People sitting in the next 3-4 rows forward and backwards can hear a standard level conversation, and whispering is not ideal given the background noise. I actually step out of the coach and carry on the conversation in one of the exit areas.
A trivial reminder: on the “other side” of the tunnel, you’re in a different country and on a different network, so beware of roaming charges especially when using web or data connections. I forgot (once).