OK, and thanks for looking into things and reporting it thoroughly.
But: it's not the point of entry into the building that's important to the problem. It is, rather, where the four telephone lines (of which only one is working) come together more or less in one place, or connect to one box (if they do. Depends on the system!)
Four wall extensions... where do wires go from them? aside from 'outward to the 4 extensions'... I mean, where do they go further 'inward' toward the telephone system box, junction box, splitter, or whatever system you have of bringing 4 extensions together.
By the way, another question: When all 4 extensions worked fine, were they all like a 'party line' arrangement, where anyone could pick up their extension and hear, or join in with, any other extension ? if so, then there is just a splitter most likely, not a true phone system box, joining the 4 extensions together.
Also, my purpose at this point is to distinguish between a phone system box or splitter box problem, and just a coincidental bad cabling problem within the walls or within the main part of the building. Once we've found out which of the two the problem is, we can proceed appropriately.
Let me know, please. Thanks.