The Partner Mail VS and the Partner ACS can both be accessed remotely.
To access the Partner Mail VS, you would simply call in on one of the ANIs being covered by the voicemail system, press "*7" once the auto attendant answers, and log in as the System Administrator using the keypad on your telephone.
Accessing the ACS processor is a little bit more involved.
In short, before you can remote access the system, you would need to make sure the ACS system has a PC card that supports remote access installed in one of the 2 front slots, download free software from Avaya's website to your computer that will allow you to communicate with the ACS, locally establish a remote username & password in the Partner ACS and do a small amount of configuring. Also, if you do remote program the system, it may be necessary to power cycle the system for the changes to take affect. Because of this, most techs prefer to be onsite and program directly from a system phone connected to port 10 or 11.
A word about remote accessing the ACS. Many modems have problems establishing a good connection and session with the ACS system. Lucent and some US Robotics modems work the best, but even then, the connection will work only at a low baud rate.
Here is a link to the R6 Install/Configure guide on Avaya's website. The information needed to set up the ACS for remote access can be found in the document.