CUBE for sure - already in the purchase bundle - wouldn't let the Carrier handle session border control!! Thanks for the tip on the Cisco-XO application note. Our interest was in things about XO SIP Trunks that might not be immediately obvious. As an example, on a transferred Single Number Reach call that rings to a cellphone, does XO support seeing the original calling party's Caller ID, or only the transferring party's Caller ID, and things like that (we'll certainly ask about things we can think of).
I'm also curious about XO itself (we've done some homework), although I'm wary of asking because any telecom service provider will have horror stories and it's difficult to tell if that's the norm (unfortunately, it is for the big Carriers), or if I'll end up turning this post into a long venting session (whether deserved or not).