Maybe try changing the Ringtone you are currently using, to something else. ie: if you're using "Over the Horizon", set "starlight" as the Ringtone, see if that solves the issue for you?
Hi, we request you to please follow these below steps once and check again.
From settings>>General management>>Reset>>Reset settings>>Reset settings>>Tap on reset.
Performing reset settings will not delete any data, it just set the settings to default.
I am doubtful that Resetting the Network settings will resolve the issue.
The call is coming through, but there is no Ringtone. They see that there is a Missed Call.