Why the Calls are not getting connected?

For Marshmallow OS version and above : 

Apps > Settings  > Connections > Mobile Networks > Network Operators > Automatic/Manual

For Lollipop OS version and below : 

Apps > Settings/More Settings > Mobile Networks > Network Operators > Automatic/Manual

• Ensure that you have not set call barring for the phone number you are dialing. If it is activated, please contact to SIM service provider to deactivate it.

Steps to check call barring : Open Phone app > More option > Settings >  More Settings > Call barring > Voice call > required options  OR

                                                       Open Phone app > More option > Settings > Supplementary services > Call barring > Voice call > required options

Method 1 

• Turn off the device.
  
• Press and hold the Power/Lock key for one or two seconds to turn on the device.
  
• When the Samsung logo displays, press and hold the Volume down key  or press option soft key continuously 5-6 times during booting until the lock screen displays. When the lock screen displays Safe mode will be shown in the lower-left corner of the screen. 

Method 2 

• Long press Power Key.
  
• Long press Power off icon
  
• Tap on Safe mode icon to turn it on. Safe mode will be shown in the lower-left corner of the screen. 

To exit Safe mode, press the Power/Lock key and then tap Restart.

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