When I re-read the parent's post I thought to myself "of course he is being sarcastic!"
But then I saw your post and it made me think. And I believe you are onto something here.
I mean, sure, probably tapping one phone is much easier physically, just connect the wires and you're done. However the point you bring is game-changer.
In ye' olden days spooks were interested in certain persons only, but now it seems that we are all fair game, and so the "easy" way of wiretapping becomes incredibly hard when you want to spy on everyone.
It's basically an scalability problem then. Never saw it that way.
Except more and more, calls are going over VoIP, which is essentially never encrypted. Even calls from one landline to another, even to a neighbor, might end up on VoIP. And in any given call, there are probably multiple resellers. Each with full capability to intercept, redirect, modify, etc. any call. And tech support is often given access to capture any call, as a troubleshooting measure.
Even companies like AT&T, who you'd think with exorbitant prices would always pay for proper direct connections, actually try to find the cheapest bidder in any way possible. For some destinations, they might a list that's 20+ resellers deep.
In short, tapping major connectivity points is probably enough to capture a lot of calls even if you place them from a landline. (Not to mention there's no real security mindset in telecom at all.)
That depends on what network you're using. If you're using a landline from the incumbent local exchange carrier, the probability of a local call going over voip is effectively almost zero. Likewise, for long distance carriers, AT&T, the ex-MCI networks Verizon owns, and the Sprint wireline long distance network (their mobile stuff goes over separate facilities; their long distance network, as well as most of the ex-MCI networks use a platform called the DMS-250, which is very much oriented to non-packet connectivity) generally don't use voip trunks for national traffic. Also, there is no least cost routing operation like on most of the smaller carriers, so there's no need to hit any sort of public network until it reaches the access tandem at your destination, at which point, it definitely isn't exchanged in any sort of IP format.
Internationally, it depends on what carrier they interconnect with and what they want. Generally speaking, I think Verizon will use more IP-based routes (usually to more expensive countries) then the other two.
By contrast, landline service coming from the cable company generally does go over voip, but only within their internal network. For local and inbound calls, it'll still hit some DSx trunks back to the phone network. 1+ long distance traffic, at least on Comcast, is definitely in IP format, and could very well even be hitting the public internet for least cost routing operations.
Back in the day all you had to do was monitor the microwave and satellite links to get all the long haul communications. You didn't have to expose yourself at all.
Now things are the opposite. It is easier to sweep up a bunch of local stuff in the form of cell phone calls but you need a physical connection to tap fibre..
Somebody told me that post-Snowden, the NSA started processing some high-value internal paperwork with fancy typewriters to prevent signals intelligence attacks. Apparently the typewriters have little signatures embedded in each letter that are unique per typewriter operator.
Every typewriter has a unique signature as has every other polygraphic machine (fax, printer, copier). This was a case always, after Media burglary (in the '70!) FBI was collecting Xerox copier printouts to identify where the burglars reproduced the stolen files.
With SS7, widely deployed in Europe and then the rest of the world, probing (tapping) was quite straight forward when the SP complied with local lawful intercept regulations.
