You can't have your cake and eat it too.
Symposium communicates with callers via MeridianMail/CallPilot. Without one of these two voicemail systems, you cannot give menu options to callers IN symposium.
If you have another voicemail system in these sites, or if you have an IVR, you can start the caller off in those systems for menuing, and then route them to a Symposium controled CDN for treatment and delivery.