How do callers know you were on the line with another caller before you answer their call?

[JAMMAY87]

Is there a signal my Samsung Galaxy Core Prime gives off to alert a caller that I am engaged in another conversation before I answer the second caller.  I say this because some callers actually say " I see you were on the phone."  At this point, I don't know how they know I was on the phone before I took their call.  I am really curious about how do they know.  If someone has an answer, I will like to hear it.

[SuzyQ]

When I call another Verizon number, and they are already on a call, the ring sound I get has a short tone at the end of the ring.

I don't hear that if they have ringback tones.