Check to see if the dummy plug is put into the headset jack of the first unit. The dummy plug signals to the first device that it is the "master" unit and that it is in daisy chain mode. If you are missing the dummy plug, you can substitute it with the plug of any headset or 3.5mm plug.

It would depend on the size, acoustics, and other settings of the room. Generally, it can pick up three to four speakers per Duet unit.

Yes, the Duet Executive (MT202-EXE) can connect to your smart phone through the headset jack, which is included with the device.