Both my wife and myself use and live Giffgaff, and one of the reason the service is perfect for us is that we spend extended periods - 3 months at a time - out of the country in the USA. Because of the your service is ideal because we ca turn Goody Bag renewals off and simply use top up credit as required for any incoming calls.
HOWEVER, your system is like "needy girlfriend" and during these periods I'd constantly emailing saying " I haven't heard from you", "don't you love me any more", "what have we done to push you away" blah blah blah blah.
So here's my idea, a simple toggle or something in your profile that can be set saying "we are out of the country from x/xx/*** to y/yy/yyyy.
That would a) let the needy girlfriend know we've not run off with someone else and b) give you the chance to even sell international calling or whatever.
Just a thought