I thought I had found an optimal solution in Possio Greta:
But I talked to the head office in Sweden, and it turned out that it doesn't work everywhere in the world. I can't remember the details now, but I had a lengthy discussion about it, and it's of quite limited use for me when I travel between different parts of the world and different countries.
Otherwise, is there any alternative to smartphone and internet, such as a small portable fax machine or any other device or other type of communication (analogue instead of digital etc.)? Telex, satellite, short-wave radio, SIP for fax etc.?