Prepaid activation and occasional use

Unfortunately that's not going to work. AT&T requires an account be fully funded 365 days a year in order for you to have service from year to year. 

But don't dispair...

    Refills of $10 to $24 expire in 30 days. Refills of $25 to $99 expire in 90 days.  $100 or moreq refill expires in 365 days.  ( it's important to note here that to $50 refills are not a $100 refill. It must be a total refill of $100 or more at one time) 

    So if you refill for a 5 gig plan with $30 plus taxes 90 days later your service expires your number is canceled in your account closed.

    Here's your option:

An alternate option would be to refill with $100 which holds your account open for 365 days. You can then use $30 refills three times and still have a $10 balance which is good for another 275 days. .

    Whenever you are not in the country you switch your account to pay per use.  The paper use $0.25 per minute or $2 per day plan will not use any Services or deplete your balance. 

Thanks for the info. So if I deposit $100, sign up for the $30 plan and switch to pay per use after 3 weeks, the $70 balance will remain untouched and is good for another 335 days. Am I interpreting that correctly?

Yes, correct! 

By the way if you live in Canada, have you tried using your AT&T service while you were at home? If you upgrade your plan, you're allowed to use it in Canada as much as you like. AT&T no longer caps it at 50% for a 90-days as they did years ago.

But I'm pretty sure the five gig plan is not eligible you would have to bump up to 15 or unlimited.

@lizdance40  Thanks again. I did see the option to use in Canada and Mexico on the $50 plan. I already have a plan in Canada so I don't need to use the AT&T plan when at home. Otherwise it would have been a good idea.