If you use Line Owner (#206-NN-3) you also have to specify the line owner inside of the Voice Mail (which is not an option if you happen to be stuck with PC card Voice Messaging)
If you use Call Cover (#208-NN-XX), the DND status of the covering extension will affect the answer rings.
Just today, needed a line to be answered by a mailbox after 3 rings, and have the messages go into the boss's mailbox by way of the phantom mailbox feature. Used a guest mailbox on Ext. 50, which didn't physically exist (it was only an ACS and a Partner Messaging in the cabinet). Call Cover caused the incoming call to be answered immediatly, with no chance to answer on the phones. Line Ownership let the line "ring" 3 times at the guest extension before covering.
6 of one, 1/2 dozen of the other.