Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
In this case you have a problem that is more difficult than you have stated in your question, as it were.
First, it's an aging system. Second, such phone systems, of that vintage, don't take kindly to inexpert or careless board-swapping... they are very susceptible to damage from static discharge, often, and also sensitive to rough handling. So if you do any work on it, be sure you know about, and completely observe, anti-static discharge precautions... and actually wear an anti-static band and grounding strap on one wrist, and also no wool clothing, no glass sheeting, etc.
Since the system is getting old, and is delicate too, I suggest (if you can observe and do what I just mentioned above in the way of careful treatment) that what it needs the most, is to be shut down, turned Off, unplugged, and to have the boards removed (and placed on either sheet aluminum or aluminum foil, or otherwise very much protected against static discharge damage) then have the cabinet and slots that the boards fit into, cleaned out thoroughly with a powerful blower or air compressor, out of doors.
Also use the compressor on the removed boards and esp. their edge connectors, *not* touching them with the compressor or nozzle, just with the air blast.
Then carefully reassemble the system again.
That might do it good.
I'm sorry - I was offline owing to an equipment failure at my usual business location at and around midday on Saturday.
And other business, etc., kept me busy Saturday evening. For this and the result, my not responding promptly on Saturday, I apologize.
But perhaps you have 'stumped' me, for I admit that the data you ask for is not readily available to me.
If you'd prefer to take your chances of another Expert picking up this case, that might have the precise information about the malfunction that you need, I will Opt Out of this case at your request, opening the case to other Experts in this category.
The money you paid for JustAnswer.com service is not 'up in smoke', I don't think.
If a phone might cause the problem, eliminate the phones one by one, by disusing and turning Off or disconnecting one of them - if this is at all possible.
If the boards are attached to different phones, then once you have eliminated the phones - all of them hopefully - then you can disuse on board at a time, and similarly see if the error stops at any time. And just possibly all this testing - with some accompanything device-reset and re-plugging - may fix the problem.
I have Opted Out, allowing others (who may be more helpful) to add their advice, if any more advice on the matter is to be had here. Good luck.