If you start missing calls from some of your contacts on iPhones, but not all of your contacts, the reason most usually is because the ringtone that you had set for that contact no longer exists on your iPhone. You can find out about this by going into the contact information of the person’s calls you are missing and scroll to the Ringtone line.
When you get to the Ringtone line it will either say “Default” or have a name of whatever ringtone you previously set for this contact. Tap on the Ringtone line to see the list of ringtones and make sure the one that is set for the contact is still listed. If it is not, choose a new ringtone for them. You will also want to change the ringtone for any other contacts that the particular ringtone was assigned to make sure you aren’t missing important calls.