Silencing Calls

I don't understand why incoming calls are silenced. I'm in business and I need to answer the phone when it rings but if I'm not literally looking at the screen, I cannot see that it's ringing. Ringer is up. There is no focus on. I searched my phone to see if there's other do not disturb settings set up, but nothing should prevent the calls from audibly ringing so that if I'm not looking at the phone I can answer. I just got a call back for service on my HVAC after choosing to let them call me back rather than wait in the queue and the phone said it was silenced! Now I have to start all over. What is going on? This is NOT acceptable. I've missed other calls that have not had any audible alert either.