score:6
For Giffgaff, regular top up credit does not expire. It lasts indefinitely, so long as your account is active.
To keep the account active, you do have to use the phone occasionally. ie make at least one phone call, or send a text, every 6 months. Giffgaff can email you monthly statements, so you could check when it was last used. You could use roaming to make a phone call while abroad, that should keep the account active, and only cost a few pence.
The Giffgaff "Goodybags" (a bundle of minutes, text or data) do expire after a month. But you don't have to buy these, you can just pay the regular PAYG rates for any usage. This would be the cheapest option if you're only making a few calls.
Upvote:2
Three UK PAYG top up credit doesn't expire even after fairly lengthy periods of account inactivity. I've had an account since 2006 and have gone as long as a year without using it.
Upvote:5
Freedompop is a relatively new option here in the UK which seems to be very suitable in this case. It has a free tier so for light use like this it doesn't cost anything. Beyond light use, it's pay-as-you-go with some of the cheapest PAYG rates available at this point in time.
I currently have a Freedompop SIM - I don't use it very much (mainly because they don't presently support transferring a UK phone number, although I believe that's coming in the near future and isn't an issue in this case anyway) so at this point it hasn't cost me anything. All the best to your Grandmother!