Thanks for you suggestions. I tried again. This description is more accurate than the one I post above:
When I press the VM key, I can hear the access number being dialed, then 10 seconds of silence, and then a busy signal.
When I manually dial the access number Google plays back the voicemail as it should.
Maybe there is some peculiar interaction between OBi and the Panasonic VM key. Has any OBi-Panasonic user successfully retrieve voicemails with their VM key? If not I can simply put the access number on speed dial.
Thank you.