This How-To covers setting up a conference on an Avaya B149 Analog Conference Handset. These handsets are supported from R7.0.
I have personally tested these steps on R9.0 and R9.1, but they should apply to any release the handsets are supported on.
Check there is a *47 Short Code in the IP Office configuration with the Feature set to Conference Add. If *47 is being used by something else other than Conference Add, create a new Short Code and use this new Short Code in the below steps.