Easy solution, I've done it many times. Use one or more pairs of the built-in SL and LS ports, connect them together (LS to SL). Program the SL station to send the desired number CALLING PARTY NUMBER and set it as a HOUSE PHONE to dial 92000 (ARS). Put a button on his phone to represent that trunk or trunk group. Now when he pushes the button he hits the trunk, which draws dial tone from the SL extension, which immediately accesses ARS. He hears dial tone and dials his number.