This can be found on "Auditory Annex Stage 1". Achievement requires you to answer the ringing phone.
Before the phone will ring, first you need to visit the music room and interact with the instruments(Thanks to PalmettoBling for pointing this out)
To find the phone, follow the signs for the "Boudoir" on the mansion walls in the Auditory Annex stage 1. When you enter the room, the phone will begin ringing. Simply interact with the phone and listen to the caller. The achievement unlocks after you hang up.