I've had a Vodafone SIM for nearly a couple of years. I got this because family in Germany are on it too, and I could get calling card access via free 0800 numbers checked before deciding to go ahead (some are blocked). But if I was starting now, I would probably go for Aldi, and try a Bizon calling card via ordinary area code numbers for 2c per minute more.
On locked vs unlocked phones, Nokias are the easiest to unlock (except recent more expensive ones), by using a calculated code you can get free from some websites such as
www.trycktill.com