Sounds interesting, but it appears to be available only in the US, and the international roaming only works for 90 days, so I don't think it really qualifies for "without bothering about new contracts, moving cities or country borders"

I dunno where you found the 90 day thing or how real/recent it is, but I was in New Zealand for about 5 months in 2019-2020 with no interruptions.

I found it here: https://fi.google.com/about/international-rates

> Important: The Google Fi Terms of Service require you to activate and use Google Fi service mainly in the United States (not including territories) for at least one day before using Google Fi abroad. If the majority of your usage occurs outside of the United States over a consecutive 90-day period, we will suspend your international data. Learn more at our Help Center.

I live abroad for long periods of time, and went from T-Mobile to Mint for my US number. It's $75 per 3 months once you pass the discounted intro period. I also get a local SIM card, and I just use Mint via WiFi calling. It has been working well for me.