The Salesforce Enterprise, Unlimited, Developer Edition, a force.com or App Cloud license is required for the call center functionality. It works with the Sales / Service Cloud and Lightning Experience.
The product is licensed per
•Salesforce user whose extension number is a result of a workflow
•Data lookup - Salesforce user who is the record owner
•Salesforce user who is assigned to an IVR workflow
•Task and Events creation - Salesforce user who is the record owner
•The Salesforce Administrator who does the setup needs a license
Example when a license is required
•User Tom is account owner and the call is transferred to the account owner
•User Alisa gets a call where the caller typed in the case number in the IVR
The phone system must be able to process web service calls (REST-API) for call distribution, data lookup and IVR).