Is *98 the access for Freephoneline's voicemail? You might need to preface it with **1 (if Freephoneline is on your obi's service 1, otherwise **2, etc).
Do you have a google voice account configured? Many things in a google account can trigger the MWI.
Are there any other VoIP services configured on the OBi?
Have you checked the local voicemailbox in the Panasonic? (By the way, KX-TGA939T is the handset, there is also a base unit (more than a simple charger) associated with it where most of the intelligence resides.)
Do you have something other than the OBi connected to the second line of the Panasonic? If so, what about voicemail on that service?