iPhone 6 Calls Keep Ringing and Won't Go To Voicemail

I have an iPhone 6 that is running the most current iOS update, and incoming calls will not go to voicemail. If I don't pick up, the call continues to ring past when my phone shows the "missed call" dialogue box. It's been doing this since a few updates ago, though I didn't realize it until recently. I tried restoring it to factory settings, but that didn't change anything.