Thanks Golden Child, that was a great help.

I found I had to add socket in front, so ended up like this for me: (I'm using windows10, so not sure if that changes things)

mame votrtnt -window -rs232 null_modem -bitbanger socket.

mame apple2ee -window -sl2 ccs7710 -sl2:ccs7710:rs232 null_modem -bitbanger socket. -flop1 disks\mb\TELEPORT.DSK

And you can see I also found that the game needed a 6850 based card for the comms. I found this add on ebay that had some clues:

Then when you hit ctrl-v, you get 'votrax engaged' and then some in game speech :-)
Talk about a narrow target audience for speech support, you need a Votrax TNT, and a CCS7710 type serial card!