Thankyou everyone. Having a system designed so the controller on the other end of the telephone can hear when the other bod is TXing to an aircraft is desirable. Can't have anyone thinking the line has gone dead. But no need for aircraft TX to go down the phone line and vice versa. I suppose any configuration is possible with modern VCCS. When a controller only has a single headset microphone for both RT and telephone, it stands to reason that some shared audio is possible, IYSWIM.