In Car Use - Speaker Phone

[M. H.]

Hi,
all my lasts phones I was able to use the speaker phone in car. I connected a 3,5 Audio cable attached to the car radio to the phones so I could hear the caller over the car audio speakers and could freely talk to him over the speaker phone. With the Redmi Pro this seems not to be possible. I can hear the caller on the radio but the caller can not hear me. Any ideas how to make that work?

Thanks.