Why Can No One Hear Me When I Make Calls Through Car Stereo

I have a Samsung S7 Active which used to connect very well with my Sony Bluetooth radio (MEXN5100BT), and make and receive calls. Suddenly, I can hear the party on the other end, but they can't hear me. The radio has a separate mic, which works fine. I tried another car stereo with the same result. Bluetooth connects, I can control the radio, for other functions in the radio App, the microphone works fine for voice commands, etc. but for some reason, the S7 is not processing the voice coming back from the stereo and transmitting it as part of the phone call.