Updating information on a very large table

We have a table with 120 million rows and roughly 300 fields. We update the
phone status of these fields every quarter. The phone update is a file of 90
million phone numbers. Basically, any phone number that matches a record in
our 120mil table triggers a record update (a single char field is changed
from 'N' to 'Y').

Any suggestions on how to speed up this process? Right now the update is
taking over 48hrs.

thanks.
.