We have problem with Snom D375 + Plantronics CS351N + EHS Advanced.
We are not able to answer the call via the head set. The same setup in combination with a snow 320 works as expected.
We have already tried to set different soundcard_event_map settings, but it didn't solve the issue.
Might it be that the ext plug of the Snom D375 isn't connected as it was covered by a plastic cap?
could you please tell us which fw version are you using ?