The baseband does not have access to the microphone. It's just the chip that handles connecion to the cell network, similar to how the wifi handles connecgions to access points.

However, the baseband can see any traffic that is unencrypted, just like the telco can. This means if you're talking over a plain old telephone service (POTS) line, the baseband can hear what is being said. However, a call to the same person over Jitsi, Matrix or a similar service would be encrypted and the contents would not be accessible to the baseband.

Your solution of using a hotspot has the advantage of separating out your telephone service from your internet connection. This means the phone number you give out to people could be a VoIP line that is not associated with your cellular service provider.