Quick reply...
Thank you. I have thought about walkie-talkie (=citizen's band radio), with selective calling, but in order for it to work, I would have to get several (maybe, a hundred, if counting my overseas acquaintances) units, for give them to all of my friends. At the same time, everybody has mobile phone nowadays - and mobile network can be found almost everywhere.
Best wishes.
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Per aspera ad astra...