Assign permissions sets to Service Cloud Voice users

Salesforce provides Contact Center Admin (Partner Telephony) and Contact Center Agent (Partner Telephony) permission sets. Assign these permission sets accordingly to the administrator and agents. 

1. On the Setup Home page, search for Permission sets in the Quick Find box and select Permission Sets.
2. From the list of permission sets, click Contact Center Admin (Partner Telephony) or Contact Center Agent (Partner Telephony).
3. Click Manage Assignments.
4. In the Current Assignments page, click Add Assignment.
5. Select the user from the list and click Next.
6. Select an expiration option of the permission for the user. 
7. Click Assign.

The permission set assignment is essential for you to access the contact center and start managing the voice calls.