Iax to iax trunk + voicemail

I have the situation which is described at the link below

(excuse me but I don’t know how send attachments!!!)

The extensions (100, 101, …) at the “nemo” asterisk server can dial to the extensions (200, 201, …) at the “trixbox” server and vice versa. Both the servers are managed by freepbx web GUI.

This is the problem: how can I dial the voicemail from an extension 1XX to an extension 2XX? and vice-versa?

Thanks in advance!