FaceTime still gets through when it’s on do not disturb

Generally my Do not Disturb button works, but whenever I’m on a FaceTime call with 2 or more people, other people are still able to FaceTime me even though I have do not disturb turned on. I have do not disturb on for a reason while I’m in a call with people because I don’t wanna be disturbed... but other people are still able to call me during these calls. Is this a problem with iOS?