Since you're the phone guru I should ask though: do you know of any other MVNOs who don't turn into bricks when you leave the USA?
There's a couple, but their rates are prohibitive or there's restrictive components, unfortunately. That is one of the downsides of MVNOs, but one of the upsides of going GSM and carrying a carrier unlocked phone over CDMA... just drop in a country native SIM card. ;)
As to that list that I know of at the moment:
Ting (you already know the rates and caveats)
Consumer Cellular (requires activation for international roaming, and one year service time)
GreatCall (terrible domestic rates, no BYOD, service geared towards old people)
Lycamobile ($1.84 a minute in Canada, 64¢ per SMS, 'nuff said.)
There's also providers that do "international" SIM cards like Telestial, OneSimCard, Mobal, etc., but most of these outfits are based out of the UK, and getting US numbers vary from carrier to carrier.
Finally, there's the VoIP shoehorned onto an Android smartphone providers like Republic, TextNow, and FreedomPop, but that's not true roaming, that's just using a WiFi hotspot in another country with a VoIP provider. This is something you can do with
any WiFi enabled smartphone and
any MVNO combined with the right VoIP provider already.
Needless to say, if you do a lot of international roaming? It's a great reason to either stick with an MNO if Ting doesn't work for you or carry a dual-SIM enabled quad-band GSM phone. It's a feature you're going to need to pay for.