If you have an analogue 5-wire audio system with speech issues (no speech, one way speech etc) then connect a test handset directly to the entry panel as follows -
Remove one handset and physically take it to the entry panel.
Using a short piece of cable (eg 2M) connect wires to 1, 2 and 6 inside the handset.
Disconnect 1A and 2 on the entry panel speaker unit.
Connect 1 in the handset to 1A on the entry panel speaker unit.
Connect 2 in the handset to 2 on the entry panel speaker unit.
Connect 6 in the handset to – on the entry panel speaker unit WITH THE WIRE THAT IS ALREADY IN THE – CONNECTION.
When you lift the handset you should have speech in both directions.
If you have 6V DC with the correct polarity at the entry panel speaker unit, but you still have no speech or speech in only one direction, then it is likely that the entry panel speaker unit is faulty.
Please make a note of the part number or take some pictures to help identify the replacement.