ajiziboc,
A SIP Device can't see the System Directory as it is not a real time service.
A SIP Device will only send messages to the MiVO-250 when it sends OPTIONS, UPDATES, REGISTRATIONS, or INVITES; i.e. it does not perform a look-up of an extension prior to making a call.
What you can do, if the SIP Device supports it, is to setup the Internal Directory of the SIP Device to know what each extension is.
Sorry,
TE