On the router with the protected phone:
dial-peer cor custom
name nobody
dial-peer cor list 555-in
dial-peer cor list 555-out
member nobody
dial-peer voice 100 voip
incoming called-number 555 <-- protected number
corlist incoming no-555-in
ephone-dn 55
number 555
corlist outgoing no-555-out
Hope this helps, please rate post if it does!
