So I looked at the community and I see the wifi app idea is on a backburner due to whatsapp etc - well could you not create a duel purpose sim that could have multiple numbers attached so it can be used in 1 phone.
My logic is this - I have been looking at ways to get rid of my land line - however the VOIP options are not that varied and usually end up costing almost as much due to the increase cost of the broadband without a phone package. So if Giffgaff could have a sim set that all have the same number (like a land line) I could put that sim into multiple phones - one I would keep at home and 2 that would go into my mobile and my partners mobile. This would be our land line and would result in all three phones ringing if the number is called.
I could answer away from home and so could the wife!, we wouldn't miss calls and either of us could pick it up. Handy for giving numbers to nursery and school, repair people etc. In addition I would still like a mobile number so I would need a duel number facility in my phone.
This way for more personal calls - I can give out my number, or for work/interview related things etc.
We could then top up the land line in the same way as we do our giffgaff mobiles,
in addition for those of us in low signal areas with the land line sim package a booster could be offered and a basic phone to keep at home with long battery life maybe with cradle doc (much like the ones you would associate with home)
Time to tackle big companies again - on the land line/line rental front
please help GiffGaff!