FWIW there was an ATT rep that posted in one of the 100 threads on the same topic here that after 5 months of NON-USE the account is deactivated and the sim is no longer usable.
There are plenty of VZW Micro (and regular) 4g SIMs on eBay (I just ordered one). Not sure if they will work for PREPAID but I will know as soon as it shows up.
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Was the PREPAID sim labeled "PREPAID" or just a regular looking SIM? If it was a regular SIM that would be good news.
I am just telling YOU what me experience has been in the past. I tend to believe very little of what any rep says. But you may do as you wish!