Thanks in advance for any help!
Zizi, appreciate the kind words and let's see what we can turn up for you.
The first thing I want to ask is how critical Verizon coverage is at your home. Are you with Verizon because you actually need Verizon CDMA coverage, or are you just with Verizon because... well... because? This is an important question to ask, as you'll see when I get to the S3 topic.
If you want to take the iPhone over to Page Plus or Selectel, you shouldn't have any problems, just be sure you pick a package that falls in line with your actual monthly usage expectations. If you don't make many calls and you've gotten the SIM slot unlocked for international use and you have good AT&T and T-Mobile coverage, you could also price TruPhone's PAYGO option against your usage needs on the iPhone. It's free incoming texts and calls, and 9˘/minute/text/MB outbound.
Regarding your daughter's S3... as it's a Verizon LTE device, IIRC, this specific Samsung device is also capable of being used as a global GSM handset as well, including domestically. No carrier unlock required as I'm sure you've noticed, just pop in a SIM, make the adjustment in the settings and you're good to go. If you have good AT&T or T-Mobile coverage in your area, this really opens up the options beyond just Verizon or a Verizon MVNO like Page Plus that will actually activate LTE handsets. However, that only addresses the physical options, and does nothing for the real-world usage wrinkles in this setup... and that is where things can get complicated or costly.
I don't know of any MVNOs that really suspend accounts to save phone numbers (P'tel is one of the most generous with a 120 day inactivity threshold on their Real PayGo account, so long as a $30+ top-up was applied first - most are only 60 days or less), as this is one of the benefits of staying with one of the big four carriers. There's ways around this such as porting the number out to Google Voice or another VoIP provider that does SMS messaging or whathaveyou to keep the number and just have it forward elsewhere, but then you're dealing with new account creation and phone activation for mobile service every time she comes back into the country, which if she switches to a GSM provider, means new SIM cards to purchase (on top of new phone numbers) every time she comes home.
Now, that said... this doesn't mean something like TruPhone couldn't be leveraged to her advantage as well depending on what country she's in, and what her own average usage numbers look like in contrast to the cost with Verizon. The neat thing with TruPhone is that there's an eight country calling area for those prepaid rates, and TruPhone's a bit more forgiving about service inactivity (so long as a text or something comes in once every 30 days or so). It's not likely to be a bargain, but honestly I'm not sure there's much greater financial benefit to be had with her setup until she's finished with school and back stateside for the long haul.
For her (and you both), it'll come down to cost versus convenience. If porting the number over to Google Voice or Ring.to or some other low-quality, zero support, heavily datamining free VoIP service and pairing it with something like P'tel's $20 unlimited talk/text and 150MB data package for four months a year plus the cost of a new SIM with every activation (which you can buy a 10 pack of P'tel SIMs off Ebay for $4) makes financial sense to you both
and doesn't sound like a huge hassle (especially
if you have good T-Mobile coverage in your area), I say do it. If you don't want to deal with it, leave her line with Verizon and have her tough it out with the higher bills until the extended periods of account suspension are no longer necessary, and then
look into alternatives for her own service at that point. Of course if she doesn't give a hoot and holler about keeping her number between visits home, that simplifies things drastically... then whatever is the best financial and usage fit is the best available option amongst the standard namechecked providers, be it P'tel, Airvoice, Page Plus, or whatever. Sky's nearly the limit on carrier options with the phone itself, excuse the Sprint MVNOs. Just note that GSM data speeds on the Verizon S3 aren't going to be anything approaching 4G LTE expectations, but it'll be plenty for anything but heavy multimedia use.
In the mean time, it doesn't mean you can't save a hunk of money now getting yourself ported out. Your daughter's the wrinkle, but it isn't impossible. It just depends on what the savings is truly worth versus her staying put with Verizon for the time left until she finishes school.