Snom D735 has a known issue where the lights on the phone won't illuminate when in use. It still functions correctly but without any lights. I talked with Snom support and they provided me with the string that needs to be changed in the template.
This issue started from Snom firmware version 10.1.127.10
Release Notes - https://service.snom.com/display/wiki/10.1.127.10+Release