I have some old Avaya stock that spun off Lucent. Avaya has been sold to private companies. How do I cash in my stock?

When Avaya was taken private back in 2007 all shareholders of record should have been contacted and given instructions on how to cash in their shares.

Check out the Avaya investor information web site: Investors - Cash Consideration (http://investors.avaya.com/shareholder/cash_consideration.asp)

You should call Avaya's transfer agent, Mellon Bank, at 1-877-296-3712 and how you should proceed to get your cash payout.

If you still have the physical shares in your possession, since they didn't hear from you they may have sent the money that was owed you ($17.50/share) to the "unclaimed and abandoned property" department for your state. You should try searching the on-line unclaimed property site for your state - see:
NAUPA (http://www.unclaimed.org/)