For the last year or so I've used
Swiftcall
Equitalk
You can type a search into Yahoo etc and you'll see their home pages.
Basically, you pay them a flat fee of £19.99 and that gets you a plan that makes all UK no.s beginning 01 and 02, and all calls to the USA & Canada day or night 'included' in that flat fee! I've found it such a money saver in terms of going the older way of getting call cards!
Check on the 'electronics' section as there's a thread about Vonage & Skype which can save even more money - of course you'll need a fast internet connection to get the best out of those services. If you are only here temporarily, I'd recommend both the companies I just mentioned, no long contracts.
Things to watch out for are, I think they both have a 1 hour call time after which you simply hang up and dial again. Also on the Equitalk one, you may find that certain calls are still charged as they may not come under the plan (times of calls i think this comes down to) Swiftcall can be a little over cautious and may send you reminders not to have 'excessive' use. Other than that, you'll save loads compared to call cards. Only other expense of course is to pay for a Phone line on which to use the service! Swiftcall also let you nominate a mobile phone on their account too, so 'technically' I suppose it's possible you may not need a landline - but It' may be part of the service that you do have a landline, check with them
hope this helps!
Cheers!! Dennis! West London & Slough UK!!