I have a customer who's UC320 is configured as a key system. They are unable dial *98 to retrieve their voicemail from the phone company hosted voicemail (why they don't use the UC's internal voicemail I don't know).
They are using a line key for a "direct" outside line. When you dial *98, the phone pauses then you get an indication that the pattern has been rejected. It's not restricted in the dial pattern restriction list. I just upgraded the UC to the latest firmaware today and it didn't help.
They have an anlog phone connected directly to their PSTN line "ahead of their system" and that's what they're presently using to retrieve their voicemail so the *98 works for message retrieval as far as the network.
As far as dialing privelage, every user is enabled for the highest level.