https://support.avaya.com/edoc/docs/partacs/pra1pru1.pdf

That is the link for your System Manual in PDF format.
Page 360 explains the procedure for line ringing options. Since I'm sure you do not have an overlay, when it says to hit the System Program button, hit the left intercom button. when it says to hit Cent. Tel Prog. hit the right intercom button.