Standard 'Call Type' field not editable and has no picklist values

You are unable to update or edit the 'Call Type' field since it is a field related to Salesforce CRM Call Center.

More details on Call Center:
This Salesforce CRM Call Center can only be activated with a CTI integration. To see if the CTI integration is set up for your customer, go to:

In Salesforce Classic

Setup | Customize | Call Center | Softphone Layouts

In Lightning Experience

Gear icon | Setup | Feature Settings | Service | Call Center | Softphone Layouts 
Alternatively, you may also click on the gear icon | Setup | Quick Find Softphone Layouts and click on Softphone Layouts under Call Center.

For further references about the 'Call Type' field and it's relationship to Call Center, please review the Salesforce Call Center documentation.

Note: Similar scenario applies to Call Duration, Call Object Identifier and Call Result.


See also
Verify Access for a Particular Field
Control User Access to Fields