Appropriate use of 'Servisol' on switches plus a clean of the pins at the BT431 plug: they were tarnished. Left for a couple of days.
Today, the test result: the 'intermittent' is now a 'permanent'.
Time for an assessment. This 'phone does not have a microphone built in; there is no battery backup for the memory; a manual will never come my way; this 'phone has a fault; this 'phone cost me nothing.
Time to part company with it. Second-hand 'phones are cheaply available in charity shops and, moreover, I do not have a pressing need for such - yet.
Al.