Assuming you are running CM 6 or higher, run SIP-to-SIP CUBE. With H323-to-SIP CUBE you need to require an MTP for all outbound calls to get early-offer. With SIP-to-SIP CUBE you can use early-offer forced on CUBE to achieve it. I'm going to base the rest of my answers upon the fact that you are going to go with a SIP-to-SIP CUBE.
Yes, place both CUBE trunks in the same route group with 'circular' as the logic for load balancing. If you want load balancing for inbound calls coming into CUBE from the ITSP, your provider will have to configure that on their end, which you mentioned they are setup for.
Yes you will need to purchase CUBE licenses for both devices.
No need for a SIP proxy here, that I can see.
Run 15.1(2)T1. Make sure you have 'mid-call signaling passthru' and 'early-offer forced' configured.
Regarding media preservation:
Cisco UBE does not support call preservation. If an Cisco UBE is located
between a gateway and a Cisco Unified Communications Manager that have
call preservation configured and the Cisco UBE is configured with media
flow-around, calls will be preserved on the gateway when the Cisco UBE's
IP interface becomes inaccessible due to a network issue or a reload.
In this case, call preservation behavior on the gateway will be the same
as the case where there is no Cisco UBE between the gateway and the
Cisco Unified Communications Manager. Calls that are "held" will not be
preserved as the Cisco UBE passes the nonstandard "do not preserve"
indication sent in the Notify message from Cisco Unified Communications
Manager to the gateway.
If you have session refresh disabled on the provider side and on CM for these calls, and if you are doing media flow-through (default/recommended), the audio should stay up if CM goes down, as long as no MTPs are invoked. It will then use the media inactivity timer to know when to tear the audio stream down.
We can do media preservation if CUBE goes down, with this feature:
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.