Missed call notifications they do not arrive on koler are managed by the system
Notifications of missed calls are managed by the system or by the pre-installed default dialer app. It would be great if they were handled by koler so that the system dialer can be completely replaced. I did some research apparently If a default dialer does not implement "android.telecom.action.SHOW_MISSED_CALLS_NOTIFICATION" receiver, a missed call notification created by Telecom is shown, this is just what is currently happening in koler.
Now i dont know how to implement it i leave you this link of this commit may be it can help you
https://github.com/TinkerBoard2-Android/packages-apps-Car-Dialer/commit/ea074a2f527d4b9b3b5fd5ceafadba83c8b8ef13
if you can implement "android.telecom.action.SHOW_MISSED_CALLS_NOTIFICATION" receiver i think that the problem is solved, unfortunately there is not much documentation about it so it is difficult