Vodafone UK Summer Roaming + Betamax = Very Good Deal

[inquisitor]

There's a an even better way. Betamax provides a Java-Midlet for mobile phones, which can be downloaded from http://gsm.voipbuster.com directly to your handset. Allthough that application is hosted on voipbuster.com, it does definitely work for all the other Betamax brands.
Through that so called "voip client" you can trigger callbacks, send SMS and check your balance oder the internet. Since only very few data volume is generated for each operation, I expect a callback can be triggered for 10-20p. That should be way cheaper than calling one of those callthrough numbers.