I use Telna Mobile and have been very happy with them. It's like 18 cents a minute in the UK, which isn't bad, but I guess wouldn't be too cheap if you are there for a few months. It might be a good backup to the tru sim though? I'd say just get a prepaid Orange sim while you are there, but I also like to have a USA number so it is easy for friends and family to contact me. Telna's CS is top rate as well.