Now that the EU telecoms market has been deregulated, there are several alternative phone providers offering cheaper call rates especially on international calls.
Within the UK, Magsys provide an excellent comparison of tariffs available through many of the UK phone providers. In some cases, you don't need to change your telephone number or wiring.
You just find a company offering the best deal for the destination you're interested in, phone up and register as a customer.
Sometimes, you need to pay a certain amount - e.g. £30 into your account, from which the cost of the calls is deducted.
In other cases, you can register and start using the service immediately - they will then send a bill every month or you pay by direct debit.
I would particularly recommend AXS and OneTel. Their rates are amongst the most competitive I have found so far.
Many discount phone providers will pass on a surcharge for calls from British Telecom payphones.
Unfortunately, the UK telecoms regulator OFTEL was not strong enough to overrule this surcharge by BT.
Check the fine print also for connection charges, service charges, expiry dates, and surcharges from 0800 numbers.
Some of the discount phoncard providers use these tricks.
AXS Telecom - join by calling 0800 358 2223.
Quote GB-49945 to get 5 pounds of free calls when you open your account!
Links to worldwide telephone directory sites.
UK Phone Net (directory enquiries)
UK Phone Information provided by the University of Warwick
Scoot - UK business listings
INTERNET TELEPHONY INTEROPERABILITY CONSORTIUM
Power Check - comparison of Gas, Electricity, Water and Telecom prices
Travel Overland - Cheap flights from Germany - Guenstige Flugangebote
Free Flights with Air Miles (UK site)