Audio Echo "Can hear myself" SIM800


I have a SIM800 used in a car handsfree. Therefore I have a “large” speaker connected via an amp. This works fine and voice quality is as expected. The SIM808 is connected as HFP to my phone which actually does the call.

My issue: The person I am talking to, can hear their own voice like an echo. If I mute my mic, they can’t hear themselves, so I suppose the microphone picks up the sound from the speaker.

What can I do about that?
I tried AT+SPE with no effect
I tried AT+ECHO, but did not know how to choose proper values. Whatever values I tried, they all seems the same. Is there a different command if I use Bluetooth HFP?

Thanks in advance.

anyone have an idea?