Log into your OBi at the IP address given when you dial ***1 (user/pass is admin/admin, or as shown on OBiTalk portal) and see what is shown under SP1 and SP2 for MessageWaiting. If either is set, uncheck the box, press Submit, then reboot the device.
If it comes back on again, then most likely the provider in question is repeatedly sending the MWI signal -- try leaving yourself a message and then deleting it.