As far as I can tell Cisco CUBE only supports unauthenticated SIP trunks, which isn’t too much trouble for Asterisk.

In sip.conf:
type=peer
host=x.x.x.x
context=trunks-inbound
allow=all
insecure=invite,port
trustrpid=yes
sendrpid=no
qualify=yes
(change ‘trunks-inbound’ to the necessary context)

In extensions.conf:
exten=>_0.,1,Set(CALLERID(num)=nnn-nnn-${CALLERID(num)})
exten=>_0.,n,Dial(SIP/x.x.x.x/${EXTEN:1})
exten=>_0.,n,Hangup
(First line is optional, and simply adds a prefix to the outgoing caller ID)

The “insecure=invite,port” is the critical part, as the source port from the Cisco CUBE appears to be dynamic unlike a tradition SIP trunk.