I'm trying to comprehend the actual meaning of this line in context — "She was my only connection, holding the cup at the other end of the string" — from a memoir book I'm reading. I assumed that holding the cup at the other end of the string could possibly be an idiom phrase but I couldn't find so either in my different keyword search in Google. Hope someone can help to clarify the meaning in this context.
Here is the context of the line from the book, I Came Upon a Lighthouse By Shantanu Naidu:
Vivaan would go out on hikes and meets to introduce himself to our new classmates as they arrived in Ithaca. It felt like I was already missing out and school hadn't even started. More and more time was spent with Tuhina on the phone as I recovered as slowly as possible. She was my only connection, holding the cup at the other end of the string.