I could not find where to press "1" in Gizmo client, but if you use native "green dot", to accept incoming call do this:
- when your tablet rings, accept/deny window pops up. Click on Accept button.
- you will hear a message "bla-bla-bla, press 1". Click on menu, DTMF dialing (or similar) - it will bring up a dialpad. Press 1 and you are all set.

Quite annoying, though. You have to be quick or it goes to voicemail.