Turn a Raspberry Pi 3B+ (Raspian Stretch) into a bluetooth headset (HSP)

I have a high end USB headset that I am trying to use with my cell phone for making phone calls while in my office. Obviously, I cannot plug a USB headset into an Android or iPhone so I am attempting a new project. I would like to plug the headset into the Raspberry Pi and using Bluez + PulseAudio (or some other software), make the raspberry pi appear to be a standard bluetooth headset so that when the phone is paired with the raspberry pi, all phone audio is routed to my USB headset. I have searched around the internet for a few days and tried a couple tutorials but I could not get it to work. Does anyone know of any examples or step-by-step guides that would accomplish this goal? Here is an example of one tutorial I tried. Granted, I am using the onboard bluetooth vs a dongle but I am using a 3B+ which has a different chipset: https://scribles.net/enabling-hands-free-profile-on-raspberry-pi-raspbian-stretch-by-using-pulseaudio/