Audio

Hi, I'm  could hear but could not be heard on a zoom call via my Samsung mobile.  I could see the Join audio, clicked on that to call over the internet, they could see my mute/unmute being turned on and off but could not hear me.

I'm not very good on these things.  On my laptop I can check audio but can not see how to do this on Android.  There is no 'up arrow' at the side of the mute/unmute on the left.  Any help would be gratefully appreciated.  Thank you.