While travelling I've been using a Telstra Prepaid Mobile Broadband modem. On purchase you get $10 of credit which I noted expires after 30 days. I added $60 more as i wasn't sure how quickly it would be used up. When i tried to connect a couple of days ago, no go. Finally discovered not only had the $10 expired (was only 3 left), they had taken the $60 as well.
Called the help desk, made a fuss and got it back after threatening an official complaint.
Just a word of caution if you use one of these. Top it up a little at a time as the 30 days starts from the last top up and your full balance is carried forward, otherwise they snatch the lot at the end of the 30 days.
Bill