On 4.1.9 and 4.0 (not sure if it is all versions) if you have SCN and the users at the site without the voicemail have Call waiting applied via user rights, you will only get one call at a time. If the user is on the phone the call will cover direct to vm. You must not set call waiting this way. Use Call waiting on the user's telephony tab.
I wasted a lot of time on this in the last week. There is a CQ. This is the fix
I wasted a lot of time on this in the last week. There is a CQ. This is the fix