Your options are fairly limited if you choose to stay with Skinny.

From my perspective the easiest solution would be to port your number to a carrier that offers roaming in all those countries -- i.e. One NZ, 2degrees maybe even check if Spark themselves have a broader roaming coverage. You can always port back to Skinny when you return to NZ. Downside is you'll lose all credit, carry over, etc when you port away and won't regain those when you port back. However, this is pretty much guaranteed to work.

There is also VoWifi as a possible workaround, but it's always seems to me a fiddly and unreliable solution. But if you won't port to another carrier then this is pretty much the last viable option and one you may want to explore. Might not be the most reliable solution.