Hi Michael!
Just for the record [or is it CD/MP3 or something like that?

] my name is Paul.
Thank you so much for those kind words!

I'm not a CPE installer/vendor by profession. I'm a Central Office technician for a CLEC/IXC, but I've installed 2 telephone systems in my church (1 is active and the other is on hot standby).
I started out knowing little about the Partner ACS & Merlin Magix, but I've been tutored by a group of guys who are tops in their field

. There is one person in particular I consider to be my mentor

. I won't name him, but he knows who he is

.
Partner MLS and Euro (I and II) phones all require 2-pairs to operate. Depending on the layout of the office, you could home-run the cables for the extensions or go for the centrally located 66 block and run a 25-pair cable back to the telephone room. As is the case with the Merlin Legend, each extension has its own jack on the Partner ACS processor and expansion modules. The only 2 AT&T/Lucent/Avaya systems that make use of amphenol connectors to any degree are the Magix and the Definity.
The nice thing about the Partner ACS is that the processor module can operate by itself for very small installations. For larger situations, Avaya sells 2 carriers. The first carrier will support the processor and 1 expansion card. The other will support the processor and 4 expansion cards.
Even if an installation will require a system with only 2 modules, the 5-module holder is the preferred choice simply because it offers better cooling capabilities than the 2-module holder does and allows for future growth.
Each holder has a backplane with slots for the male edge connectors on the rear of each & every module to be inserted. Power is also distributed to every module via the backplane.
Avaya offers a selection of auto attendant/voicemail systems for the Partner ACS. For small applications with minimal features, consider the Partner Voice Messaging. This auto attendant/voicemail is actually contained on a PCMCIA card that slides into a slot on the front of the processor. One version offers 4 mailboxes while the other will offers 12 (actually 16, but the available recording time will be very small so figure on 12 max).
For larger applications, consider the Avaya Partner Messaging. This will support up to 200 users and up to 100 hours of total recording time. This voicemail is contained in a Partner housing and would take up a slot in the 5-module carrier.
Out of box, the R6 processor will support 3 lines and 8 extensions. The R7 processor will support 5 lines and 9 extensions. Common expansion modules currently offered will add:
3 lines and 8 extensions (308EC)
0 lines and 12 extensions (012E)
There are other expansion modules for partial T1 service and SDSL connections, but they are not used very often. Still other expansion modules are available but they are old and, in some cases, obsolete.
Having a Norstar Meridian at the place where I work, I've been able to see how both systems program and the Partner ACS is easier hands down. The Meridian is closer to the Magix/Legend in complexity.
A few more thoughts: (lol)
If you go to some of the other tech help sites (Tek-Tips and Sundance for example) you will notice posts that deal with problems that have plagued the Partner ACS R7 processor since it was released for sale a few years ago. Avaya issues patches to fix one problem, but creates others in the process. The "K" software has been the most stable so far. "O" is the latest. It adds some nice features, but it too has had issues.
Also, don't be shy about seeking the services of a local Authorized Avaya Business Partner if the task looks daunting. They can install, administer and support a system very easily as well as train you and others in basic programming ....AND.... if a problem arises, it will be nice to have someone else face the "firing squad"!
