I've been having a similar problem with my new Pixel 9a, running GrapheneOS. (I've only had this phone/this carrier for a few weeks.) So far, every time I've lost a call this way, I've been holding the phone to my ear (often, while also walking around the house.) When it happens and the call ends, I look at the phone and see that I'm now in Airplane Mode. (I didn't notice that until once when I tried to call the person back and got the "get out of Airplane Mode first" error message.)
My service provider had me give them a specific date/time of a call that was lost. They told me the call ended from my Pixel 9a phone, as follows: "RELEASE_CAUSE, cause=1, text='User ends call". They are advising me that the problem could be a device issue with Pixel/Graphene.
Two days ago, I had 2 long calls with different businesses (one had already complained about all the dropped calls lately too). For these 2 calls, I decided NOT to hold the phone to my ear. Instead, I held it face up, out in front of my body, and tried to keep it parallel to the floor as I was walking from room to room/floor to floor in my house (looking for what these businesses needed me to find). I did not lose either of these long calls. Maybe that's just a coincidence, but now I'm wondering if this issue is due to some software bug related to the position of the phone (portrait/landscape toggle, for example). Or, maybe I'm accidentally activating something with my ear/cheek when I hold the phone to my ear, although I see no obvious functionality I could be accidentally triggering from that particular screen. Ugh.