Why not open that capability to an API so that somebody who is operating some sort of forwarding service to run spam detection before forwarding?
And like you say, they do need to prevent their system being used for sending spam. But if they're not going to provide a way for honest parties to avoid sending spam, they need to be reasonable about how they react to spam reports.
I would think sending spam would be more if we allowed people to sign up for a our service and then people used it to send spam.
But these are all internal communications.
They already detect outgoing spam. Good customer service in this case would be to detect that a customer is attempting to send spam and blocking it and potentially notifying them about it.
Sending one and the system might not have detected the spam. It's not visiting the link. But send many at once the system gets curious.
That OP mentioning "reprogramming the system so it doesn't forward messages" makes me think that they are receiving the incoming messages programmatically via webhook and then using Twilio API to send a new message with same text to new number (their employee).
In such a case, they do have a point in the process where they can implement some kind of filtering or processing (maybe stripping or obfuscating URLs?)
It's an incredibly common use case to forward text messages from one incoming line to another number that is controlled by your company, and it's impossible to tell at the point of receipt (when it's still in Twilio's network) whether it's spam or not before forwarding it on.
It's the same exact thing that you might do if you have a [email protected] email address; it probably sends incoming emails to a number of different inboxes, even if those incoming emails are spam (and even if there actually is a spam system in place that might block some percentage of incoming messages, and Twilio doesn't even provide that.)