So I installed eOS on my Mi A1 and everything except this bug seems to work really well out of the box. I was using Lineage 16.0 before.
Its a really annyoing bug that limits my PHONE to a tablet kind of. My Speaker is working usually, as are Signal Calls for example. But whenever I call someone or get a call via Phone network I cant hear a thing. The call connects, the other person can hear me but I dont.
Right now I don’t have my Mi A1, but it worked fine with /e/OS a few weeks ago.
Try flashing the latest official firmware with Android One, and then flash normally /e/OS https://bigota.d.miui.com/V10.0.19.0.PDHMIXM/tissot_images_V10.0.19.0.PDHMIXM_20200311.0000.00_9.0_78b42319da.tgz
It is from the Xiaomi servers (bigota.d.miui.com / hugeota.d.miui.com). Actually it is not the last firmware of the Mi A1 but it is the last leaked (that Firmware is March 2020).
They have recently modified the official website of mi.com and I imagine that as long as they do not leak a new version of the Firmware, it will not be able to download from the system that they have put now (https://c.mi.com/es/miuidownload/detail?guide=2)
Located a more updated version, also with an official link: https://bigota.d.miui.com/V10.0.23.0.PDHMIXM/tissot_images_V10.0.23.0.PDHMIXM_20200707.0000.00_9.0_b49c1d0e32.tgz