Hey, thanks Deltron! I appreciate the info. Unfortunately, I was not able to get those instructions to work as written. However, with slight adjustments I was able to use those SLT codes to work for me. The trick was to dial an external party first before being able to use the 438 or 664 codes. Please see my revised instructions if it will be of any help for people in the same situation.

1. Press "On/Off" button. (You will have an external dial tone)
2. Dial your first external party.
3. Once connected, press "Conference" button. (You will hear 3 quick tones then an internal dial tone)
4. Dial "438". (You will hear 3 quick tones and then reach an internal dial tone. The code allows for the next party dialed to be automatically placed on Park)
5. Dial "9", then your 2nd external party.
6. Once connected, press "Conference" (You will hear 3 quick tones then reach an internal dial tone.)
7. Dial "664" (You will hear one long tone, and all external parties will be connected).
8. You may choose to now add an internal calling party by pressing "Conference" (You will hear 3 quick tones then internal dial tone.)
9. Dial extension of your 3rd party
10. Once connected press "Conference" to connect all parties.

Thanks again! Case Closed!