One of my favourite giffgaff features is the option for customers to choose which calls and SMS bundle (in giffgaff parlance, goodybag) they want on a month by month basis. This also allows customers to choose the experimental option from giffgaff labs.
This month I picked the (bizarrely named) Hokey Cokey goodybag. For £5 I get 300 SMS and 60 minutes of calls, however the clever bit is for every minute of calls I receive I get an extra minute added to my call bundle. giffgaff are taking advantage of the fact that they get paid by the originating operator to deliver a mobile call to their customer, so they return some of that termination charge to their customer.
giffgaff recognises that many customers have several mobile numbers and want a customer’s giffgaff number to become their main mobile number. Well, it worked for me!