Vodafone UK Summer Roaming + Betamax = Very Good Deal
 

The Vodafone UK summer roaming promotion is discussed elsewhere.

Essentially, when you roam in the listed countries, incoming calls are free, outgoing calls to the UK and the country you are in are 20p per minute, and 38p per minute to all other countries.

While there was some discussion on what the rates are, a review of these two web pages makes it clear that I have accurately described the rates above:

Vodafone Passport ? mobile phone calls when roaming abroad

Vodafone Simply Call Charges - Pay As You Go Price Plans - Vodafone

As has also been discussed elsewhere, the various Betamax services (poivy.com, etc.) have local landline numbers in various places that one can call into and then call at the cheap Betamax rates. Betamax has 4 such numbers in the UK.

Assuming Vodafone does not block access to these numbers, one can roam in Europe, call the Betamax number for 20p per minute, and then call another country, frequently for free if you already have a Betamax account.

Also worth noting, Vodafone has an anonolous pricing structure in the UK of 20p per minute for domestic calls, but 5p per minute to Canada, China and the US. Thus, if one is in the UK, and wants to call the UK, it is cheaper to call a Betamax access number in the US for 5p per minute and to then call the UK number from the Betamax gateway.

BTW, all of the above is on the assumption that the correct caller ID is called from the Vodafone SIM to the Betamax gateway in the UK or distant country.

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.