Also, you can print out the telediscount number you'll be needing

http://www.telediscount.co.uk/accessnumbers.php

This will let you call most places in the world for pretty much the same price as calling London.

I've memorised 0845 244 244 2

it's a good catch all for most international numbers

dial 0845 244 244 2 - wait until the female voice begins - then dial 00 + country code and number followed immediately by #

Only downside is that you still get charged (albeit very little) if the number you are calling doesn't answer or is engaged.

Assuming your call is answered, this will let you talk to someone overseas for several minutes for 30 pence.

I agree that using a credit card to buy phone services is risky. Use your credit card to buy goods instead from someone who can factor in some change from your purchase (or see if you can buy on behalf of the person behind you in the queue and get change from them - people are usually pretty helpful once they can see you're not out to screw them over)