I have been using the "free" extension to keep our number active without any difficulty for quite some time, but if you fail to extend your number at least every 60 days for free then the only way to use the "free" extension again is first to re-activate your number with a TopUp account. If you do not have a TopUp card account or are not on the island then there is an outfit here in the States that will obtain an account for you or add funds to an existing account. Chances are your number will not have been reassigned during the interim. Here is a link to their site:

http://buychippieonline.com/xbccard.aspx

Also, bear in mind that just because you have a positive pre-paid balance in your UTS/Chippie account does not guarantee your number will remain active if you have not been using it. The purpose of using the "free" extension or TopUp account is to keep your number active, that is to reserve it for your use and not allow it to be reassigned to someone else. After re-activating your number with a transfer from your TopUp account you should be able to resume using the "free" extension service.