Hi there and thanks for visiting the community.
(Taken from the link below).http://giffgaff.com/boiler-plate/terms
12.1 Your SIM card will be disconnected if your account has been completely inactive for a period of six months.
Your account will be considered ‘active’ if, during this period, there is any ONE of the following:
At least one call, SMS or MMS made to another number (For technical reasons, this does not include emergency calls (i.e. 999/112) and SMSs or calls to giffgaff services (i.e. 2020, 43430)) At least one connection to Internet, At least 4 received calls of more than 10 seconds, At least one top-up /purchase (credit or goodybag), The receipt of Payback points (for recruiting new members or participating to the Community). If your SIM Card is disconnected, you will lose any remaining credits/balance on your Account. If you wish to be re-connected, you will be given a new mobile phone number.