In order to prevent a number of tooltalk security problems, a new method of tooltalk authentication was introduced.
This method uses a shared secret authentication system, using $HOME/.TTauthority. This new authentication scheme is the default.
Both systems that don't share home directories and systems with older tooltalk client software won't be able to connect.
In the case of different home directories/user ids, you will need to transfer TTauthority information. This works much like $HOME/.Xauthority information. The new command to use is ttauth(1)
In the case of older client software, you will need to install the relevant ToolTalk patches. These are available for Solaris 2.4 onwards.