The CUCM by default provides different ring cadence on the IP phone for on net and off net calls but its not possible to configure different ring tones.
The ring cadence doubles in frequency for off net calls. The 'Chirp 1' chirps twice when the call is external(offnet) and chirps only once when internal.
Use the Gateway Configuration window field, Call Classification, to configure the gateway as OffNet, OnNet, or Use System Default.For information on on-net ,off-net call classification see the system guide.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/7_0_1/ccmsys/a08gw.html#wp1100223
