If you install Apple Telecom 3.0 or Express Modem 3.0 after installing the System 7.5.5 Update you will not be able to switch on your Express Modem. To fix this incompatibility, run the System 7.5.5 Update again. The System 7.5.5 Update updates your Express Modem 3.0 to Express Modem 3.0.1.
NOTE: Apple Telecom 3.0 shipped on some new Macintosh systems and is not presently available from Apple for download or purchase.