As B*Cat says, this thread is growing many heads, and stretching memories ....
Therefore, my guess, and it's only a guess, is that T4 would be connected to Earth for old 'Earth Calling' systems
And it was even specified for domestic circuits before all the public exchange equipment was standardised onto the current 'timed break' recall. One old telephone, of 1980s vintage, that I have given to my brother in law for use as his bedside phone has a recessed switch underneath to select the recall method which will be used.
I believe that we may be merging memories here ... B*Cat's recollection references (timed and earth)
recall which AFAIrecollect was a P(A)BX facility. The switch he refers to allowed the same apparatus (great old word) to be used either on a subscriber (another great old word) line, or on a P(A)BX as an extension, in which case the earth recall allowed the P(A)BX operator to be alerted e.g. to come back and transfer the call elsewhere. With further (A)utomation the operator was replaced with a further dial tone and/or the ability to dial another extension directly. Later, as he said, this was replaced by
timed break recall instead.
'Earth Calling systems' may recollect something even further back -
shared lines, in which the same line was shared by two parties. For billing reasons, the normal 'off-hook' loop seizure could not identify the calling party on a shared line, and therefore 'earth calling' on either the A or B leg did so instead.
Also, and I appreciate I could be way off beam here, some (at least) of the references to 'surge arrestors' might be conflating the old pre-1959 lightning strike idea (open copper drop-wires then) with 'surge suppression' as in
bell-tinkle. Remember at the ~1983/4 changeover, backward compatability was required with existing loop-disconnect phones. The difference between extension wiring before then and after is more or less predicated around using 'high impedance' bell (more likely tone) circuits with no need therefore for 'bell tinkle (or surge) suppression', as opposed say, to the good old GPO 706 'modern' telephone [1], with a low impedance bell and the need for 'bell tinkle suppression'. The bell tinkle suppression circuit in the 706 used a
thermistor as a 'surge suppressor'. It is (at least) possible that older NTEs may have had to have a similar device for compatability with older phones. It is worth noting that 'newer' phones at that time would still (theoretically) have caused 'bell tinkle' if used in loop-disconnect mode, if it wasn't for the fact that the high impedance of the tone ringer did its own 'bell tinkle suppression'.
Confused? You will be!
[1]
http://www.samhallas.co.uk/repository/po_docs/ep_draft_telephones_1_1.pdf
