iphone Dialup from iPhone AS and iPhone UK Ltd.

From iphone.no:

Call our access number from any telephone or operator
What do I need to get started?:
Any telephone will allow you to call our access number

Register your details
Log in and buy call credit
Call our access number 08450340880 or 020 80902800
Benefits
Free setup
Very low international call charges
No monthly charge
Works from any telephone or operator
Buy credit with your credit card or by SMS
Register the telephone you usually call from to save entering your PIN Code
Call history details on your web account
Ring directly from your phonebook without entering the access number