This was a thing among my parent's friends and relatives when I was a child in the north of England in the seventies. People would say something like "give me two rings to let me know you got home safe". I have no idea what the cost of landline calls was back then.
It also features in Iain Bank's novel Whit from 1995. The main character is a member of a Scottish religious cult, who mitigate their exposure to "ungodly" technology by using a message-passing code based on the number of rings before hanging up. Its a long time since I read the book, but I seem to recall some detail about it having to be error-tolerant because the ring-tones that the caller hears don't necessarily match those heard by the recipient.