The 18D phone has a MIC/HFAI button. It acts as a hands-free intercom. So, press the button, and have any other extension dial *10,*11 or whichever.
See if that works for them.
It may not give ultra high quality results in a noisy setting, but, it may suffice.
As an alternative, would a simple set of two way radios/walkie talkies be doable?