There are an increasing number of products which need GSM connection (Alarms, Remote Switches, Smart Appliances etc) - it is a real pain that the SIMs in these devices get cancelled after 6 months unless a call out is made
Why not introduce a special category of SIM with a fixed charge every 6 months to cover the cost of holding a live phone number, but where the SIM will never be cancelled for inactivity (as long as the 6 monthly charge is paid)
It would cost nothing to implement and could be achieved by setting a single flag (presumably you could use the field you are using to track inactivity but just set it to 999999 and you already have the ability to manage recurring billing
Not only would this increase revenues for GiffGaff (I'm assuming you'd charge more than the 5p it costs me to send a text every 6 months at present to keep my numbers live) but it would also drive sales - the first network provider to offer this would be the provider of choice for alarm companies and home automation everywhere!!