Not so! With my 3 PAYG I earn free minutes which nearly always expire before they get used, and 0800 etc.. doesn't generally count for deduction from your allowance. It depends entirely of your phone company, so
you need to check the tariff. But it's certainly not going to cost you more - so there's no point knocking it!
And that's another money saving tip: shop around and see if your tariff is the most suitable one for your usage. Millions don't, and the phone companies make millions in extra profits because of this. Consider carrying multiple SIMs to get the best deal at all times.
An unlocked phone allows you to keep multiple SIMs and swap networks to optimise your usage, and also to take advantage of special offers.

Locked phones on contracts generally work out a lot more expensive in the long-run than simply going to a shop and buying an unlocked one, and then selecting the network yourself. And, at the end of the contract, you could get charged extra to unlock it too - so you've got to add that to the cost of ownership. Locked phones are worth less on the second hand market too, so you're not going to pass that cost on to someone else.
Compare the cost of buying the phone on credit with the cost of signing the contract, and disregard all the free minutes you realistically know you will never use. It's unlikely you are going to change your usage pattern significanly, so learn from yourself! Also take into account that you might want to change the phone, or the tariff, before the contract runs out. Don't be pushed into signing; take your time and do your homework before you agree to a contract. Generally speaking contracts are not the best option - unless someone else is paying the bill!
And..
if your contract has expired take the thing straight to a phone shop and get it unlocked, even if you have no immediate intention to change networks. Inertia makes money for phone networks.
And here's something else you should consider before signing a contract:
http://www.bbc.co.uk/news/10185221 With PAYG when the credit is out the credit is out, until you chose to renew it.