If you don't have the manual for the Avaya Partner system, you'll need that in order to properly program the automated attendant and to assign different greetings for other phone lines.
I've attached a link with the manual for your system, that you can download free (save it on your computer for future use and reference). Printing the whole manual will take quite a bit of paper, but you can do it if you wish.
The information that you're looking for is in section 4, 5, and then testing the system in section 6.
The only tips that I can pass along are that you need to make sure that your sysem is properly setup to begin with, otherwise, many of the features and functions won't be able to be programmed or changed. If you have any doubts about your system settings, begin again and go through the set up procedures before you attempt to program new functions and features.
It can be a bit daunting to deal with these systems at first, but if you take the time needed to read through the manual, you'll be better aquainted with everything this system can do. Then programming and making changes to it, won't be all that difficult.
Of course the other option is to hire a telcomm systems tech to come out and perform the work for you, but then you're at their mercy everytime you want to make a change. It is a pretty straight forward system to work with, so please, set aside some time to read the manual and you'll be fine.
I hope you find this Very Helpful and best regards!