Review on UGREEN Wireless Bluetooth 5.0 Car Adapter - Portable Bluetooth Receiver for Music Streaming, Sound Systems, Speakers, Headphones - Hands-Free Car Kit with Microphone by Jason Porter

Contrary to what the seller or others say in the Q&A, this (and presumably all of those "bluetooth receivers") does not use the built-in microphone of connected headsets. In fact, most of them are not intended for use in headsets, but rather to be connected to the 3.5mm input of a car stereo. It has a built-in microphone, so you can use it as a car radio adapter for hands-free calls, but the quality of the microphone is pretty mediocre, to be honest. I'll keep looking for a device that can fully convert a wired headset *with a built-in mic* to Bluetooth, but so far it's been useless. If anyone knows of such a device, please share in the comments!