You can try setting up Extension Mobility for the users that will be specific to the users. Hence, if the employees leave simply delete their User Profiles and create new profiles for the new employees. Refer below for more information:
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/8_6_1/ccmfeat/fsgd-861-cm/fsem.html
Edit to the O.P. If the above does not suffice your requirements then you can also try to configure the phones using BAT which will be most suited for this. Refer below for more details:
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/bat/10_0_1/CUCM_BK_CDB9A6A4_00_cucm-bulk-administration-guide-100.html
Regards
Deepak