iPhone not ringing with VoIP calls

About a week after setting up my Ooma Office and the mobile app my iPhone quit working right. It appears that when I receive a call from a person using VoIP my iPhone only gives a notification that I missed a call with no caller ID. When I call it from my VoIP Ooma Office phone I hear a message that “the wireless customer you are calling is unavailable”. If I use a landline or another cell phone the call goes through normally.
My cell service provider cannot figure out the problem. I removed the Ooma mobile app, but my iPhone still has the same problem.
Has anyone ever had this same problem? If so, how did you fix it? Any help would be greatly appreciated. :slight_smile: