When already on a call and someone else tries to call me it goes to voicemail

Hello everyone,

For some reason if I'm on a call and another person tries to call me they get sent straight to voicemail. I even tested it myself by calling a friend and then from my house phone call my iPhone and it went straight to voicemail. It appears the phone rings only when I'm not already on an ongoing call. Does anyone know why this is? Or how to fix this issue? Thanks!