I remember using the second definition in elementary school in the early 90s, before cell phones were on common use, long before they flipped open, and even before they had extendable antennas. I suppose they might have been a cordless landline, but I always assumed it was a corded phone. The “call me” message, then, wasn’t about being able to see someone but not hear them except in very specific circumstances; instead, it was implied to mean “call me later.” It could be used as a way of flirting, or it could be more platonic. I suppose it could also be used in a business setting, though I wasn’t really old enough to know.