Not directly, as the flight mode really prevents any radio communication (it takes a bit to shut down as it sends a NAS Detach Request first and waits for a rep

Not directly, as the flight mode really prevents any radio communication (it takes a bit to shut down as it sends a NAS Detach Request first and waits for a reply for a few seconds). However, there is also WiFi Calling - in that sense, your phone establishes some connection with the cell network. However, I don‘t think any user data may travel on this bearer, but there might be some edge case where this is possible.