Have you tried swapping 2 phones to see if the problem is a phone? To try that, swap 2 phones of the same phone type otherwise you will default the key programmimg.

To change the general mailbox phone will like I mentioned require you to connect a pc to the SVMi card serial port and program it.It's hard to say how they are getting the messages to the general mailbox without seeing the routing screen.How do callers get to the general mailbox?