Here's how I would do it.
1) Restrict it to inhouse onlydialing
2) Hotline it to the extension you need it to ring to.
To restrict it to inhouse dialing, assign it a unique Class of Restriction. i.e. 22. Then, in the Maximum Digits Dialed form put a 4 in COR 22 if your digit extension numbers four digits long or a three if they're three digits long.
Next in the Associated Directory form, create new entry, and make hit "hotline" to the number you need it to.
Now if someone picks up the phone it will ring the front desk, security or wherever you need it to.
Ralph