Currently this is only exposed on the full SA administration interface on the user input page (or via a number of admin tools such as BulkEdit and AudioText Manager) - no end user tools are out there that expose the user input key mappings at this point since it requires users have access to call handlers/subscribers in the system which, of course, requires admin access.
There was the concept of "personal call handlers" kicked around at one point that would allow individual users to setup their own call handlers with transfer options and the like but that would not live in the "shared" space where all the other handlers/subscribers lived (and thus not cause numbering plan conflict issues and the like) - they'd then be able to map their own user input keys (zero included) to one of their handlers to do transfer to whoever or wherever their restriction tables allowed them to. This never got off the design table, however, and is just a requirements definition in the library at the moment... the best thing to do here is to ping your account team and ask them to stick this (or something similiar) in PERS as a feature request against Unity such that it gets on the list of things considered for future versions.
