I'm not sure that's entirely true. When I leave Thailand I make sure to buy a top up card and, when I next arrive, simply do the top up at the airport. Any unused credit expires after, in the case of a 300Baht card, 30 days as I recall but the number is reactivated as soon as you next top up.
In Lao PDR, on the other hand, when you top up after being away for any time, the credit is reinstated. Likwise I buy a top up card before I leave so as to be ready for the next visit.
That's 1-2-call in Thailand and M-Phone in Lao, by the way.