Hey guys, I'm stuck!

So I have a list of Staff, Telephone Nos and the Office they work at. They are sorted into an alphabetical list as shown below:

STAFF: NUMBER: OFFICE:
Alfred 0001 East
Dave 0002 East
Mark 0003 West
Zippy 0004 East

Now on another sheet I have a cell with a dropdown list to select which Office then another cell that has a drop down list for members of staff, I want this second list to only show the staff at the office chosen in the first cell, any ideas?