no mention of what IDS cabinet you are working on but you should check in your operator terminal (or programming terminal) and be sure that your voicemail options are turned on within the IDS. After you enter your password, you will see them on the top line of the main menu in the IDS programming. Should say VMS or VMS VMI(depending on your application) on the top line, without those options listed you will not be able to interface with the voicemail. If the IDS cabinet recently lost power, it is possible you lost these options. After checking your connections and other hardware as mentioned in the above posts, RNA when dialing 436 is usually an indication of this.
Tj