Refer here: http://www.cisco.com/c/en/us/td/docs/ios/voice/cube/configuration/guide/vb_book/vb_book/vb_9655.html
Support for Multiple Registrars on SIP Trunks
The Support for Multiple Registrars on SIP Trunks on a Cisco Unified Border Element, on Cisco IOS SIP TDM Gateways, and on a Cisco Unified Communications Manager Express feature allows configuration of multiple registrars on Session Initiation Protocol (SIP) trunks, each simultaneously registered using its respective authentication instance. Beginning with Cisco IOS XE Release 3.1S, support for this feature is expanded to include the Cisco ASR 1000 Series Router. This feature allows a redundant registrar for each of the SIP trunks, which provides SIP trunk redundancy across multiple service providers.
Prerequisites
Cisco Unified Border Element
•Cisco IOS Release 15.0(1)XA or a later release must be installed and running on your Cisco Unified Border Element.
Cisco Unified Border Element (Enterprise)
•Cisco IOS XE Release 3.1S or a later release must be installed and running on your Cisco ASR 1000 Series Router.
-Terry
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