How do I play a different message to an answering
machine in the same call session?

You can do this by selecting Machine message in the Call Wizard, then recording
a separate message that will go out to answering machines only. Read more about
these options in the Call Wizard section. Now when
you start this call job, if PhoneTree® detects an answering machine it will leave
the message you recorded for Answering Machines instead of the Standard Message.