**You will also need a SIM card (you may get this free) and a 'Top-up' of £10 to get you started.
If you then take it back for whatever reason (in my case it didn't work) for a refund, you will not get the £10 back.**
This as been the practice of places like Argos for years. Thought everyone knew about by now, apparently not.
I assume the reason behind "you will not get the £10 back.", is due to a third party (service provider) transaction, when the voucher is printed out. Perhaps also similar to an expiry date on some vouchers, that some people never notice, if they do not activate the mobile and voucher in a certain time frame.
I've had PAYG since about 1998, and still have the same number as I've transferred the SIM, and the credit, plus the £10 creit, from phone to phone. The only time I've changed SIM is from a full size one to a mini.