Hi There,
Since you didn't include any information about your system I will assume that your phone is registered to Unified Communications Manager.
If that is the case, then it sounds like your line is configured to call forward no answer (CFNOAN) to your cellphone. You can request your administrator to change it so that your CFNOAN goes to your voicemail instead of your cellphone.
You could also try using the self care portal (if you have access) to disable your call forward to your cellphone. If the portal is enabled (and you are on a version of CUCM that supports it) you can try navigating to https://<FQDN or IP of CUCM>/ccmuser and logging in. If you can login to the portal you should be able to adjust your call forwards from there.
Also please consider moving this topic to the appropriate section of the support forums (IP Telephony). https://supportforums.cisco.com/community/5961/ip-telephony
Please let us know if this helps.