Skype on a smartphone is of limited value -- my carrier certainly has a contract clause forbidding the use of web-based messaging and VOIP when using their mobile network (can only be used through wireless). In other words "Don't you dare to use your unlimited broadband access to deprive us of charging you for phone calls".
I couldn't see anything about that in my PAYG T&Cs. I did use it at one point over a poor quality 3G connection (on my iPhone), it was awful. It only works well when you've got a good WiFi signal.