iPhone drops call without ringing

Troubleshooting condition if 

1. You just upgraded your phone, and

2. Using an old SIM card that you had in your old phone, and

3. If you can make calls out from your phone fine, and

4. Incoming calls show briefly on the screen and disconnect without ringing, and 

5. Incoming calls appear as missed calls without phone ringing

6. The caller hears nothing their end, then connection attempt terminates

Then it's likely that you may have a 2G, 3G or 4G card in your 5G phone. Upgrade your SIM.