How to disable iPhone from ringing while in do not disturb

My iPhone is ringing while I have it in DND and silent mode and I have made sure of all the settings as far as not allowing calls from anyone and making sure they are not “emergency contacts” etc. It happened twice while listening to music on Bluetooth speaker and once while wearing headphones. Very disturbing as I am a massage therapist and cannot have this happening while working and then while I was in a deep meditation with my headphones. Totally frustrating that I cannot disable the phone ringing! 😖