Before starting you should reset your phone(s) if they have been previously registered to another CME/CUCM:
https://realtechtalk.com/Cisco_IP_Phone_How_To_Reset_To_Factory_Settings_Instructions_CP_9971_8800_8900_8845_8851_8841_8831_7961_7960_7945_7942_7941_7940-2212-articles........