I was recently a guest staying at an elderly woman's large house.
The room I slept in was full of junk, but what caught my eye were envelopes with stamps from the 1950s/1960s. The "From" name was the woman's late husband, and they were all from two African countries that were then part of the British Empire.
I had heard her talk about when her husband was working in Africa, but until I saw the big pile of letters I hadn't considered that this was the only way the young couple could keep in contact when someone's work required long periods of international travel.
No need to go as far as Africa. Basic landline telephones spread into poorer/more rural parts of Europe only in 1980s and 1990s. Until then it was either taking the bus to see people in person, or sending a letter....
E.g. quick search revealed this pamphlet from 1988 Spain, showing that ~30% of households did not at time yet have a telephone: https://www.telefonica.com/en/wp-content/uploads/sites/5/202...