Originally Posted by TheBigM:
“Vectone is a pretty good mobile network for making international calls. Also check out Lebara mobile.”
Lebara do a nice set of international-focused rolling SIM-only contracts, which are good if you need calls to one country specifically. For £15 (special offer at present seems to be £10) you get a certain amount of minutes to your chosen country (200 for me, to the U.S.) and 100 UK minutes:
http://www.lebara-mobile.co.uk/paymonthly
Other than that, if you want to call multiple countries, their PAYG rates are really rather competitive with the major UK networks (5p/min landline, 10p/min mobile) and they have a London call centre and use Vodafone as their network. I've switched to them (my partner lives in the U.S., I'm in and out of the country, and I'm off there permanently in a few months) and find them pretty reliable with a good sound quality and decent reception.
Before Lebara, I used Giffgaff with Localphone, but the sound quality just wasn't there for me. Localphone is super cheap and they have a fantastic interface and great customer service, they're good for business calls etc, but when the love of your life sounds like a compressed robotic voice underneath a bunch of hiss and crackle, you start to think about paying a bit more. (Localphone also has a weird delay issue when calling the UK from the U.S., which it doesn't have the other way around. It's odd.)