I have had an iPhone 3GS for several years. It is factory unlocked so works on any network. In that time I have used it on O2 and GiffGaff. I've recently tried putting a newly activated Vodafone SIM into it and am getting "No Service". I've spoken with Vodafone and they have confirmed the SIM number is correct and it is fully activated. Do I need to do anything in iTunes when I change network? I just swapped the O2 SIM to GiffGaff previously and it just worked - but I'm aware that GG use the O2 network. So am wondering if I should just be able to stick the Vodafone SIM in and it work (in theory!)?