I have "stutter dial" for PSTN voicemail message waiting. My OBi110 works like your Linksys unit. My cordless phone attached to the OBi110 does not flash a light when there is a message waiting on the analog PSTN line. You can, of course, on the Obi110 pickup the phone, hit # and get the pstn line actual dial tone and hear the stutter dial.
I have an external caller id box that I got years ago from Pacific Bell that I have cabled on the PSTN line to show a flashing message waiting light that I can see from across the room. Looks like you may be able to buy that box or similiar brands on EBAY or from Amazon.