Years ago there used to be a service called 'Orange Everyphone'. I've just re-discovered that it does still exist, but it's only available to business customers.
The idea was that you could divert your calls to a landline or another mobile on the same network, free of per minute charges.
You could also dial a short number from your handset e.g. 330, or 07xxx xxx330 from a landline or another mobile to set the divert up, if for example you had left your phone in the office, or at home. - I have noticed a similar idea has recently been posted up in the ideas forum!!
Obviously, the termination fee GiffGaff receives for calls to it's mobile range dwarfs that of the termination fee for diverting to a landline, hence I feel this service is not only useful, but actually viable.
Orange also charges a one off 8p(ish) charge for setting these diverts by charging to call the 330 short code number, or if setting up from the landline they receive termination revenue by using 07xxx xxx 330.
This could make GiffGaff money whilst providing a service users are asking you for!
**Please kudos if you like this idea, and would like to be able to divert your calls to landline numbers for free!**