Through G Mail with your laptop you can call any UK landline for 2p per minute or a UK mobile for 7p per minute providing your laptop/netbook has a speaker and microphone. You have to open an account and have a balance in it. I put 10 quid in and opted for automatic top up at 2 quid via my debit card. I found this after Travel Republic kept me on hold on my mobile for 7 minutes whilst confirming my return transfer.
p.s. Why's my avatar not working boss?