One person with whom I've emailed before said to "drop me [them] a line". I always thought the intention of this phrase is to call the person, specifically by phone (via telephone line). It seems this person means to email them. I do think it's possible they mean to call them. Is it a widely accepted use of this phrase to mean "contact me" in general? Is this usage common among certain groups of people? Am I wrong in associating this with phones in particular?
In the U.S., I think I'm safe in stating that, in the vast majority of cases, 'drop me a line' implies written communication, and never implies calling via telephone or any other means. Before the Internet was invented, 'drop me a line' always meant 'write me a note/postcard/letter'. Nowadays, the phrase could also imply emailing.
I can't speak for British English, or for speakers from other parts of the world.
Edit: I also just discovered that this is a duplicate: 'Drop us a line' - letter or phone call?