'Drop us a line' - letter or phone call?

According to the Free Dictionary, dropping someone a line means sending them a short message.

Is this correct? I always thought it meant phoning someone, the line referring to a telephone line.

Solution 1:

It refers to a line of text in a letter, so it means sending a short message (maybe with just one single line of text).

(If you drop the line during a phone call, that means you're hanging up)

Solution 2:

History says the telephone was invented in 1876. I imagine A. G. Bell would be amazed at this Ngram (could he see it) if, indeed, it refers to telephone communication!

Solution 3:

Drop me a line invites someone to write something to you: a message, a note or a short letter. It probably has to do with the lines that comprise a piece of writing.