Calls failing on some contacts

If you are experiencing intermittent silence or drop or the call itself is disconnected during voice calls on your iPhone, there are a few things you can try to fix the issue:

1. Poor signal strength can cause dropouts or silence during a call. Try moving to an area with better reception or switching to a different network if possible.
2. Restart your iPhone --> Restart your iPhone, iPad, or iPod touch - Apple Support
3. Force Restart your iPhone --> Force restart iPhone - Apple Support)
4. This may help too. Go to Settings > General > Transfer & Reset > Reset > Reset Network Settings.
   1. When you reset network settings, previously used networks and VPN settings that weren’t installed by a configuration profile or mobile device management (MDM) are removed. Wi-Fi is turned off and then back on, disconnecting you from any network you’re on. The Wi-Fi and Ask to Join Networks settings remain turned on.
   2. To remove VPN settings installed by a configuration profile, go to Settings > General > Profiles & Device Management, select the configuration profile, then tap Remove Profile. This also removes other settings and accounts provided by the profile. See Install or Remove configuration profiles on iPhone in this guide.
   3. To remove network settings installed by MDM, go to Settings > General > Profiles & Device Management, select the management, then tap Remove Management. This also removes other settings and certificates provided by MDM. See “Mobile device management (MDM)” in the iOS Deployment Reference.
   4. Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
5. Check if there is a software update available for your iPhone. Go to Settings > General > Software Update.
6. Check if your audio is not connected to external Bluetooth speakers.
7. Few other reasons why your device keeps Muting / Dropping / Disconnecting Calls or encountering Call Failed.
   1. Your device's cellular signal Is weak.
   2. It's Raining (Cats & Dogs) in your area or in the area where your device is hooked.
   3. The Cell Tower is Overloaded / Heavy Traffic (Telecommunication)
   4. You've misunderstood your signal strength
   5. You are moving and the signal weakens while changing from one tower to another
8. Other culprits may include the following:
   1. Bad Weather: Precipitation can leak into phone cable lines and cause disruptions. Lightening, even at a distance, can disrupt the connection.
   2. Wireless interference: Power lines, radio, or television. transmitters, transformers, and other electronic devices may cause noise on the phone line. --> Resolve Wi-Fi and Bluetooth issues caused by wireless ...
   3. DSL services: The high-frequency signals coming from DSL (digital subscriber line) devices sharing the phone line can create problems for other technology connected to the line.
9. Try to turn on Airplane mode, wait for 10-15 secs, and turn off airplane mode.
10. Check the health of your device's Battery. Change if below 80%
11. Take the SIM out and Put it back again, Change your SIM, if it is damaged
12. Time to contact your Carrier / Change if required