SIP Broker is a free service that provides unlimited VoIP calls to 2000 SIP networks and millions of people around the world. No signup is necessary.
There are even PSTN access numbers in 20 counties including UK London & UK 0845 you can use if you dont have a SIP phone ! http://www.sipbroker.com/sipbroker/action/pstnNumbers
For example just dial +44-20-7100-6747 *011188888# to hear a test message