Hmm... three Telephone sets 'D' Mk.V (which look like the Canadian version with cast-aluminium rather than the British pressed steel cases), something I don't recognise but may be a small buzzer switchboard[1] for the telephones, and an Apparatus LoudSpeaking on the right, with two 6V 85AH accumulators to power it. It's the 4-channel version, so probably for a 4 x 25pdr gun battery.
Nice photo.
Chris.
[1] Somewhere I have a "3-line switch unit", allegedly for Artillery use, but it has Kellogg keys and doesn't (I think) look like the one in the photograph. So: a phone for each member of the team plus the ALS for the Gun Position Officer, and presumably lines to the FOO and the unit commander?