FYI, i figured this out myself... there were two issues for me. hope this helps someone... my phone is a polycom ip450, but #1 below should be true for most any phone.
1) fixing the indicator not coming on:
on elastix, on the extension, there is a 'mailbox' that defaults to something like firstname.lastname@example.org
. if you change that, the VM indicator will STOP WORKING. leaving the default does seem to work, but changing it to just '101' (the extension number) works also.
2) POLYCOM SPECIFIC:Message button not accessing voicemail box
pressing the 'message' button on my polycom phone defaulted to calling the extension back itself (which is of course busy). why??? only polycom knows how this default could ever be remotely useful... to get the message button to call voicemail so you can check it, you have to change the 'callback mode' to 'contact' and 'callback contact' to '*97' (or however you dial voicemail. you can do this several ways. 1) on the 'lines' of the web interface all the way at the bottom. 2) on the phone itself in the menus or 3) in the [mac_address].cfg configuration file using <msg msg.mwi.1.callBack="*97" msg.mwi.1.callBackMode="contact"/>
fyi, i engaged the elastixs chat support for help on this as the forum community seems a little sparse here and the community a little discouraged which worries me as a newbie. they couldn't figure it out, but watching them try did help me find the solution right after i disconnected with them. on another note, for what it's worth, while they couldn't find this particular problem (which was more polycom-related perhaps to give the benefit of the doubt?), the support is immediate and reasonably priced ($70/hr) and i found it to be very good. i will use it again whenever i'm stuck...