It should be possible to test this kind of things more systematically, for example to play some chosen test-sound generated by some other sound source, play that into the L5 mic during a call that gets picked up in a voicemail, then download the resulting recording from the voicemail and compare the sound that came from the call to the original test-sound. Then do the same test for some other phone(s) to see how the L5 behaves compared to other phones.
On second thought, probably best to start by testing the mic separately by just recording something using gnome-sound-recorder or similar and check if there is a problem with some frequencies. (To see if there is an issue with the mic itself, not necessarily related to the phone call functionality.)