@lipu, Thanks for the graph and your assessment of calling. We have so few people with Dogwood giving their impressions of the phone.
I have this same problem using speaker mode during calls in LineageOS 16.
I wonder if this is a universal problem or a problem specific to your carrier. If you don’t mind me asking, which cellular network provider are you using?
There is not yet software support for MMS. See: Librem5 MMS Question
Do you have any estimates with the screen on?