Seems fewer and fewer airlines are allowing guitars as carry-ons nowadays. It used to be that you could stow a guitar in a soft case in the overhead bin or in the closet, but now it seems most airlines expect you purchase an extra seat for the instrument (yeah, like that's ever going to happen! LOL.)
The U.S. airlines I used to fly into BKK always used to allow guitars as carry-on luggage, but, unfortunately, it seems most of them use code share flights with other airlines (e.g., ANA, Korean Airlines, et al) for the final leg of long haul flights into BKK nowadays.
My solution, when flying with guitars with bolt-on necks, has been to disassemble the necks and pack both the body and the neck in a checked bag.
I've never run into any problems doing it that way.
I've read anecdotes from some travelers about how they were still able to finesse their guitars on board in spite of these "purchase an extra seat" policies, but I'm unwilling to take such chances.
I lucked out when I first moved to Thailand as I was able to ship all of my guitars via USPS to a relative's APO address at domestic U.S. postal rates.