Now we use Cisco IP telephony based on Cisco CME. Cisco CME is installed on 2821 router with IOS C2800NM-ADVENTERPRISEK9-M, Version 12.4(24)T7. A SIP trunk is configured between CME router and telephony provider.
We plan to upgrade our telephone system and begin using Cisco CBE6000. I study the documentation and in "Cisco Business Edition Unified Communications and Collaboration Solutions for Small and Midsized Companies", I find that for SIP trunk to works in CBE6000 a Cisco Integrated Services Router with Cisco Unified Border Element is required. Or other quote from this document "Public switched telephone network (PSTN) connections using SIP is available through any Cisco Integrated Services Router voice gateway".
In this regard there is a question: what is the functions of this router when I make a trunk between CBE6000 and telephony provider SIP gateway and whether it is possible to use existing Cisco 2821 router as such router.