Hi Maheer,
Please check the below:-
1) Install the client on the sever and check if you can login with the same credentials.
2) Please check if the operators IP phone MAC is added to the application user in CUCM.
If the above have negetive results, please collect the Wireshark packet captures and OPR agent logs from the client PC and attache it here.
Regards,
Ankoor Bek