I use onetel for overseas calls, put up with BT for local but mainly use my mobile anyway so don't have many. Just dial a 1877 access code before the full number and you are taken out of the BT charging loop and onto onetel. NZ is 6p a min 24/7 from memory, US about 3p or so. Ireland around the same.
Although with f2s for broadband my old onetel email address is working fine after a year and downloads into Outlook. I go into it every few days via their webmail when not at home - "mailzone", so maybe that has something to do with it.
Think you can get slightly cheaper overseas calls than onetel but not much cheaper. Used to see them advertised all the time when I used the tube a lot.