The Metropolitan Police in London U.K. use a system called ship to shore, though I don't know if that is it's brand name. Anyway this apparently costs around £250 and has two black boxes one you plug into one of the tails of the aircraft intercom whilst the second box plugs into the socket of the helmet of the ground crew. Permanent on and no extra wiring.
Off-shore we used to use an icom which was tuned into the company frequency and that's how the ground crew communicated to us.