Sim Activation: To get your sim activated you need three things: 
    - Your giffgaff micro SIM card -  order one here

    - A giffgaff account

    - An unlocked phone: If when you put a giffgaff micro SIM card in your phone and get an error message and no coverage sign, it's likely that your phone is locked to your previous network.

    - A payment method ready - you can pay using most major UK credit or debit card, or pop to the corner shop and get a giffgaff top up voucher.
    GiffGaff also accepts o2 top up vouchers - as we run on the o2 network.

    - Make your first top-up when you activate your giffgaff micro SIM: Got your SIM card and an unlocked phone? Now you need to activate your SIM online. You'll be asked to insert the code that's printed on your SIM card, create a username and you'll have to make your first top-up or goodybag purchase (£10 minimum).

What is the catch?: - First, giffgaff don't give micro sims, so I am given them nicely cut free of charge. Why? because, once you activate your SIM will get the free £5 pounds credit  (when you top-up your first £10 pounds first top up only) and I will get 500 giffgaff points.
GiffGaff has rolling contract, so you can leave whenever you want if you are not happy with the service

Can I use my iphone with GiffGaff? : Of course! and you could take advantage of some of the unlimited data plans. You can view/download the leaflet here: http://dl.dropbox.com/u/1428694/giffgaff_iphone4.pdf

Can I keep my number?: You can transfer any UK mobile phone number to giffgaff, provided you have fulfilled any contractual agreement you may have with your previous Service Provider. Once you've activated your SIM, you can bring your existing phone number to giffgaff. It's a quick and simple transfer process. You first need to ask for a PAC code from the network you're leaving and then fill in the form to transfer your number in the giffgaff website

You will get £5 free credit once you activate you sim, and make a £10 pound top-up. order free giffgaff micro sim