Are you talking about buying extra credit for international calls or roaming outwith the EU?
From what I gather, anything bought is for the month and not taken forward. (unless you've used up the value before the month is out)
I use Giffgaff and put extra credit on my phone a few years ago when going abroad. The remainder of that credit is still on my account, so unless they have changed the terms the unused credit stays on your account.