Google Mo-Call. I have a Vodafone PAYG sim but I signed up to Mo this summer. The tariff I’m on with Vodafone charges me 50 cents for a local call up to 59.9 minutes (Vitamina 60 x 1 24h) but I can then get connected to the UK via Mo to a landline for 2.5 pence a minute or Mobile at 20 pence a minute depending which network I am calling in the UK. I tried it this summer and whether I spoke to the UK for 5 minutes or 55 minutes I got charged 50 cents from Vodafone the other charge was based on the usage rates as I’ve mentioned above.

Also as EmmaLouUk mentions you can use Skype, they also do Skype on a mobile I belive too.