Just found this...
GiffGaff piggybacks the O2 network. Sim cards are free, you can either use their standard prices, as follows...
To giffgaff mobiles, texts, call forwarding and video calls Free
Calls to other UK mobiles and landlines, call forwarding (p/m) 8p
Texts to other UK mobiles and landlines 4p
Voice mail (per call) 8p
Mobile Internet (until the 28 FEB 2011) Free
Picture messages to giffgaff and other mobiles 16p
Video calls to other mobiles (per minute) 50p
Call forwarding to other mobiles and landlines (per minute) 8p
0800 numbers Free
After 28th Feb, mobile Internet will be charged at 50p a day (if you use less than 2.5MB/day you'll pay less than 50p and if you go over 30MB you'll be charged extra at 20p/MB). But all goodybags that include unlimited Internet will continue including unlimited Internet after the 28 FEB 2011.
Or purchase a goodybag...
£5 goodybag - unlimited texts
£10 goodybag - 250 mins, unlimited text & unlimited internet
£15 goodybag - 300 mins, unlimited text & unlimited internet
£20 goodybag - 600 mins, unlimited text & unlimited internet
£25 goodybag - 1500 mins, unlimited text & unlimited internet
"Unlimited" is for personal mobile use. Mobile Internet allowances exclude PC use and tethering.
If you recommend a friend who then goes on to activate a sim (I've not put a recommend link above btw...!), they give you 500 points. 1 point = 1 penny. You can have this back as cash via paypal, airtime or donate it to charity.
To be eligible for Payback you must have an active giffgaff SIM, have topped up within 3 months before 31 MAY 2011, have a positive Payback points balance, not be in breach of any of our standard Terms and Conditions