Are you able to call the SLT extn numbers directly for the voicemail eg 200 or 210 whatever you created earlier?
The voicemail should answer with the default greeting etc

You have to set the telephone system type in the Teleco VM as option 2 for DTMF integration or type 3 for SMDI integration.
Then you need to select the pre-configured dialplan from 1-0 depending on digit lenght and mailbox range.
If you have the installation manual this will be straightforward to do if you don't have the manual then it is best to get someone to do this for you otherwise you will be spending lots of time trying to get it to work.
Finally you need to set the LTN ports to match the created SLT VM extension numbers.

Do yourself a favor and get the Teleco AGM (Admin Graphical Manager) software if you want to do any changes to the Teleco VM it will be much easier to manage.

Personally I would rip it out and put in an LVMU and it will be more reliable and easier to maintain moving forward.

Regards
Carl