The 3T was possibly more sensitive.
When used continuously by operators on public exchange switchboards, the receiver may be too loud for comfort. Under these circumstances the sensitivity should be reduced by shunting the receiver with a Resistor, Carbon, No. 4, 150 ohms. When used at PBXs this shunt is not required since an 180 ohm series resistor is provided in the operator's circuit.
jack units for headsets were usually used on Key and Lamps.
https://www.britishtelephones.com/head2.htm
https://www.britishtelephones.com/jackunit1.htm
What we'd do without Mr Freshwater's magnum opus website, I do not know.