... The phones at this location have been anywhere from 3 to 5 minutes behind the pc...
I would change time servers... if you are using an internal Domain time server right now, it might actually be off (as your above quote alludes to), I ran into this at a customer site once before and the Mitel was syncing to their Domain Server but that server wasn't syncing to any outside time server. Changing the time server to "us.pool.ntp.org" fixed the issue, if you need a different zone, pool.ntp.org has lots of zones and specific country areas you can choose from to sync to.
If the end result is to get the Mitel and the PCs to have the same time, they all need to use the same source and it has to be within a certain tolerance of the public, global network clocking system.
If you still have problems after syncing to a reliable clock source, this could be a digital trunk issue where the carrier is not synced up to a public network clock and their clocking source is off significantly, this would be very odd and unusual but the theory is solid. Or the 3300's RTC is defective, also very unlikely but a possibility but if you getting this deep I would contact tech support.
