Firstly I really don't know much about this, just spotted it yesterday.
Bottom left of the tesco mobile page it says enter mobile number for some great offers. Enter your tesco mobile and you will be sent a text message with a pin code to enter.
Entering that code will take you to your profile page. Somewhere on that page (I'm sorry I can't be precise as it has disappeared from my profile page, presumably because I have already sent off for it) it says click here or tick this box to be sent a £20 mobile phone upgrade voucher.
If my memory serves me right I'm sure it said for use in store only.