These tend to have high per-minute call costs, but if you don't make many outgoing calls, they can be exceptional value for money. Even if your voucher has expired, you can still receive incoming calls and make emergency calls, though keeping some credit on the account makes sense. Orange JT are available with or without handsets, but the bundled handsets tend to be old, inferior models. Their handsets seem expensive, because there is no subsidy, but the selection available includes good ones. These deals are the networks trying to appeal to Joe Public, and much of the new subscribers are on these deals.
Similar deals are still available, but the prices varies from month to month.
There are several companies offering such deals. I have listed some on the SIM-Only dealers page , but this list may be complete or inaccurate. The inconvenience of this is partly eliminated by using a Personal Number. Don't automatically assume that a Sim Only is the best value for you, however. You hang up and the switchboard in the USA rings you back on your mobile.