Paul,
Registration has nothing to do with placing outbound calls, it only affects how FS delivers inbound calls to a registered user. Whether or not FS will challenge incoming invites for credentials depends on the values of "auth-calls," "auth-all-packets," "accept-blind-auth" and various ACLs.
For example, it may be the case that you have a proxy in front of FS that results in FS actually seeing the internal IP address, combined with an ACL that allows FS to trust any calls coming from the internal network.
I think it will be hard to help more without seeing your profile configuration.
Best,
Colin