Am I right in assuming that the Tesco value sim card is simply a way of getting the PIN access code to dial through the 0207 number, but you need to make the actual calls to the 0207 from another phone which has free minutes (as landline calls from the Tesco Value sim card cost 15p per minute);
http://direct.tesco.com/q/R.200-8770.aspx
Some landlines (i.e. BT Free Weekends) allow free calls to landlines on the weekends so can use the 0207 number on the weekend, and use the PIN obtained from the double Tesco credit?
Further info on the other country calls charges are on
http://www.planettalkcard.com/
This offer can work with any sim card which has introductory free credit. I'm sure there was something out there that was giving £10 free on new PAYG phone purchase, which cost £15 and if buying via Quidco, it was working out to be just a few quid. Need to find the free sim cards and start obtaining these free PIN codes