To handle this via the basic IVR functions of Symposium (so without the Access ports) it is more difficult:
GIVE IVR MAIL_DN WITH TREATMENT 123456
Where MAIL_DN is a global DN variable which is the prompt maintenance one)
In MerMail you should setup a Voice menu with ID 123456
Fill in the options you would like to have, as action assign CL and for the calling number a new CDN (Of course also point the Voice Menu 123456 in the Voice Service DN Table to the correct entry)
This CDN should be aquired in Symposium and be routed to the same script as it came from.
The insert a section which allows this CDN to skip the welcome message.
The disadvantage is of course that this requires many CDN's, but it works.