The feature is called Automatic Line Selection. You can read about it on Page 5-3 (Acrobat page 171) of the Partner ACS Installation, Programming and Use manual, which can be found at support.avaya.com.
The programming is described as follows:
1. Press <FEATURE> 0 0 at extension 10 or 11.
2. Enter the number of the extension to be programmed for Automatic Line Selection.
3. Press **.
4. Press the line, pool, or i buttons in the desired order. The valid entries for Automatic
Line Selection depend upon the type of extension:
? Key Extension: Outside system lines, Left Intercom (the factory setting)
? Pooled Extension: Pool 880 button 1, button 2, Left Intercom, Right Intercom (the factory setting), Pools 881- 883, individual lines
5. To exit Automatic Line Selection, press **.