I've seen that message after calling voicemail and, afaik, it's just informative, not an error message. The problem is accessing voicemail. You could try disabling and re-enabling it, but I don't know whether this would delete the message - what you do rather depends on how important they are. To turn voicemail off, dial 1210 and to turn it on, dial 1211. Did you get a missed call message in your call log? If you did, you can call back, of course, though you won't have if the phone was out of coverage of switched off.
If a restart doesn't work, the service may need to be reset at system level and Live Chat (Contact Us at the bottom of the page) should be able to hekp with that. I think this probably will lose the messages.
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