Hi Peter,
You can either configure with a proxy line or with a shared line. Both the procedures are available in the below link:
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/10_0_1/ccmfeat/CUCM_BK_F3AC1C0F_00_cucm-features-services-guide-100/CUCM_BK_F3AC1C0F_00_cucm-features-services-guide-100_chapter_01011.html
HTH
Rajan