avayapbx

I have a digital station (type 3904) with 5 digits number. I want to assign MWI using key 16. Everytime i assigned MWK to key 16, i receive this error.

Message waiting key (MWK) requires Message Waiting Allowed (MWA) Class of
Service.

How do I create MWA, and also accord to the manual to assign MWK:

xx MWK yyyy mwc-1
Message Waiting Key
Where: yyyy = DN.
The DN can be up to 4 digits, up to 7 digits with
Directory Number Expansion (DNXP) package 150.
With the Network Message Service feature
equipped, the NMS-DN can be up to 13 digits.
On the M3902, key 5 is reserved for MWK or NUL.
On the M3903, M3904, M3905, key 16 is reserved
for MWK or NUL.
basic-24
On the i2004 key 16 is reserved for MWK or NUL. mit-25

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What id the DN number, how do i create this number?

Thank you,

Khoa