airplane mode

Q: What is yc?
A: There is a mysterious nation exists in our universe, it's hard to find. I once met a citizen of that nation, we talked much, changed numbers with each other in the end. After I came home, I JUST can't remember the content of the conversation, but I always have good memory. I open the note on which he wrote his phone number, ha, weird, there is no number, just 2 letter.
Q: y and c?
A: yes.