I has an iPhone 4 with Verizon service for the past 5 years without issues making calls from my house.
I upgraded to the iPhone SE last week and now all my calls drop constantly and people on the other end of the line say they can't hear or understand me.
What's going on? The service is worse now.
Go to Settings > Cellular > Enable LTE > Check to see if "Voice and Data" is enabled.
If it is, try setting it to "Data" only and see if the situation improves.
If the situation does improve, turn the "Voice and Data" function back on. Then, go to Settings > Phone and enable Wi-Fi Calling. Wi-Fi Calling will allow the phone to call over your home Wi-Fi especially when it thinks the LTE reception may be a bit sketchy, and allows the phone to hand off between Wi-Fi and LTE without dropping the call.