silent ringtone

I need a silent ringtone. I have emotionally unstable relatives that sometimes call over a 100 x day, but other times are fine. I need to be able to not block them, or b/c of work turn my ringer off.

Why does my iphone 6s not have a silent ringtone, but I have friends that have the original SE and even 5 and 5s's that still have it?

Again, do not waste time posting about the ringer off option, block contact list, etc. Not looking for a look around, only for the why this is on some phones and not others and how I can get it.