The tone is generated by the communications service which it's currently working on.
If you travel to France by Eurostar, while carrying a phone that uses Vodafone in the UK, the phone will lose its signal (from a Vodafone mast in the Folkestone area) as soon as you enter the tunnel. When you come out of the tunnel the phone will quickly lock onto the French SFR network (so that anyone calling it will hear the continental tone).
It's the same if you travel by ferry. Your phone will first go out of range of the UK system, and then lock onto a partner company's network in France. As soon as it does so anyone calling you will hear the continental tone and they'll have to pay 'overseas rates', while you'll have to pay roaming charges to receive calls.
Chris