SPID= Service Profile IDentifier.
When you order an ISDN-BRI line from the local telco, they should give you a SPID associated w/ each B channel of the BRI.
Here is what the Avaya online expert says about SPID programming,...
How to program SPID (Service Profile Identifier).
All interface terminals used with the Lucent Technologies 5ESS switch or the Avaya DEFINITY Enterprise Communications Server must have a unique Service Profile Identifier (SPID) number recorded within the terminal. A SPID is not required when the terminal is used with the 5E4 switch. This number is entered into the terminal via the terminal's key pad. A terminal connected to the DEFINITY ECS typically uses the terminal's 4-digit extension number plus two additional digits as its SPID. These two additional digits range from 00 to 15 and represent the terminal identifiers A through P, respectively. A terminal connected to the 5ESS typically uses the terminals 10-digit telephone number as its SPID. The 5ESS switch calculates the SPID automatically upon completion of the service order input process. Obtain the SPID from the System Administrator.
! CAUTION:
The SPID number identifies your terminal to the network and should not be changed unless your System Administrator so indicates. Changing the value could result in loss of service. Use the following procedure to enter the Service Profile Identifier (SPID) in the terminal and key telephone.
Press the Shift/Select or Select button. Message lamp comes on.
Press the Mute button followed by key pad digits 'I' and 'D' ('4' and '3', respectively). The present value of the SPID is shown on the display.
Enter digits from the key pad for the new SPID number.
NOTE:
To erase a SPID, enter '*'.
To save a SPID, enter '#'.