nomi-mobile let you make international calls from your mobile and home phone at near VOIP prices !
Just replace your UK SIM with a £5 nomi SIM then buy a top-up (from any of the outlets below) and start making calls to other UK mobiles/landlines/etc @ 13p/min or to International destinations (for example calls to Turkey are only 5p min). You can also pre-dial the UK access number 080 0008 6805 from a UK landline and pay even lower prices ! There’s even a charity version of the SIM available !
VOIPPIX 