It's mostly illegal to make human or automated marketing calls to numbers in the US that have signed up for the national Do Not Call registry. The problem is that huge numbers of illegal calls still reach numbers on the Do Not Call registry. Most of these illegal calls have no connection to any real business; they are instead trying to defraud people. I have often been called by "Microsoft Support" to alert me to Windows problems that need fixing. I don't have any Windows computers.
[S]ince mid-2015, a consortium of engineers from phone carriers and others in the telecom industry have worked on a way to [stop call-spoofing], worried that spam phone calls could eventually endanger the whole system. “We’re getting to the point where nobody trusts the phone network,” says Jim McEachern, principal technologist at the Alliance for Telecommunications Industry Solutions (ATIS.) “When they stop trusting the phone network, they stop using it.”
https://nymag.com/intelligencer/2018/05/how-to-stop-spam-rob...
Trust is one element. Common weal, the sense of acting in the common good, is the more critical element.
Trust here is a mechanism, rather than and end goal itself. Through the undermining of trust in a system for the public benefit, the common good is harmed.
Your counterexample becomes far less difficult when phrased equivalently:
Through the undermining of trust within a system opposed to the public benefit, the common good is increased.
TL;DR: you've confused ends and means.
Though generally, common good is extremely reliant on a strong social trust fabric.
(As to the question of whether or not "common good" / "common weal" is an appropriate goal value of a system, that's another discussion. I've elected to adopt for now the general idea that it is, but there are varying points of view on this point, and cases in which there is no globally beneficial outcome. Most especially where some are disadvantaged such that others, any, might survive. See also: "lifeboat ethics".)
The robocalls I receive are all using fake caller IDs. At that point, it's fraud, isn't it?
I wouldn't mind robocallers if they used a real caller ID. If that were the case, I could just block them and be done with it.
The goal of, effectively, all robocalls is fraud or value-extraction from the system. It works only because of the scale possible.
Small-scale operators (unless hugely and widely distributed) don't exhibit these characteristics. A fundamental problem of robocalls is the scale of operation itself.
Scale matters.
Countermeasures which disrupt at-scale operation whilst protecting small-scale activities are net beneficial.
If you're on well-legislated do-not-call list, a person is not allow to call you unsolicited for non-personal purposes.
You got a number I can send you the answers to? \s