Hi,
I've also installed Ubuntu touch 20.04 on my OnePlus 6.
I also have to problem that people who I call only hear electric static sounds.
Mic seems to work fine when using the Audio Recorder App or when recording Videos; although even there the mic sometimes won't work all of a sudden which only a restart or reset will fix.
I was able to talk without problems on the phone when connecting my bluetooth headset.

I installed Ubuntu Touch on the OnePlus 6 via the "ubports-installer_0.10.0_win_x64.exe".
And one thing I was wondering about was, if it is normal that the phone is calling itself OnePlus6T when connecting my headset via bluetooth.