Hi,
To achieve this you have to configure Extension Mobility feature for End-Users.
This will allow users to make call with his own number by logging in any Voip Device (within the cluster) through Extension Mobility feature.
Refer the below link for more detail:
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/8_0_1/ccmfeat/fsgd-801-cm/fsem.html#wp1386026
Regards,
Venkatesh