Just remember that an SLT device will not be placing a call on hold, only keysets will be placing calls on hold. For this reason, only keysets need to have their T/R connected behind the line card. A combination of off-hook current flow sensed on the station side T/R plus the A-lead going open is what causes the line to go on hold.
Current flow on the station side is ONLY needed for this purpose. SLT devices need not be drawing current through the station side because they have no means to open the A lead simultaneously while in use.
Then, we need to refer back to the potential for bridged ringer load affecting the line card's reed relay's current limitation.
Western Electric's 400H line cards were designed to address this argument (among other things). Any device going off-hook on the station side T/R without A leads present will immediately result in an on-hold condition when using this card.
It was never done that way on 1A2 key systems in Bell System territories.
Funny, that's exactly where I learned this, from New Jersey Bell, NY Telephone and C&P Telephone installation practices in the mid 1970's through the 80's.
Also, many brands of line cards included polarity guard for the station side (Some San/Bar, Brand/Rex and ITT400TPL and WE400H). Ringing voltage on the station side would be unavailable. This was an FCC thing to prevent DIY's from getting shocked with consumer-level 1A2 system sold over the counter. Yes, there were phone stores that sold 1A2 systems and San/Bar made them, designed for DIY use. Sales of these required fully-protected line cards where the T&R were electrically isolated from ring voltage and the line's 48VDC.
If I am so far off the mark, I urge anyone here to explain the difference between an RJ12 and RJ13 connection. I'm fairly confident that you will find that I am correct.
I have printed documentation if anyone is interested in a fax. I don't have it stored in a format where I can e-mail it.
