Voiptalk are offering a new SMS callback service, it works by calling back to your mobile then connecting you to your destination. You must first sign up for their silver pre-pay package then register your mobile phone number. Looking at their website it seems that T-mobile customers in the Uk will get the best value as their callback rate is 20 p/min versus 25 p/min.
If you spend a lot of time abroad in a particular country say for example India, if you purchase and Indian mobile you could save on calls back to the Uk from your mobile using this service. The India mobile callback rate is 7 p/min plus the Uk destination rate of 1.4 p/min comes to a total of just 8.4 p/min. On the Voiptalk site their is a calculator click this link to View the Calculator.
To find out more general information about the service use this link