Hi Sharon,
I stopped using the number for about one or one and a half months, so nowhere close to one year. The number on the topup page appeared as valid (i also checked with an older prepaid sim and indeed i had an "invalid number" error). When the payment was processed, i just never got a confirmation text for the topup and my credit stayed at 0 euros. The last time i put money on it was the 20th of march ; i also used the online topup website, which worked without problem then.
Best,
Lucas