Hey, i had a same problem, after updating firmware on the KD-8003, i resolved the problem when my phone was not calling me when someone was on the door. But the quality got terrible! It was ok only in the configuration window on the hikvision batch configuration window. I did restored to factory settings (from the intercom menu - KH-8350) and readded the intercom again, i had to activate, change IP and then restart the KH-8350.
TLDR;
When poor quality after firmware update (2.2.6x), restore door station to factory settings, readd and restart intercom.