Depends on how they're "dialling in".
If this is a connection via the Internet to your LAN, that's a bit more problematic - but providing a username and password specific to each person who is likely to dial in means a) you can log/audit the connections and b) pin down the access those people have to only those areas they require.
If dialling in via a modem to a specific PC - disconnect it from the LAN (if it's not the one with the confidential data) or see above.