The sheer fact that it routed to a line that rang is enough for them to know it's a good number. Their auto-dialer could be configured such that answering it increased the cadence of calls or something such as that. But the absence of the call being routed to a not-in-service/no-longer-available response is enough of a signal for them to know it's a good number.
Source: I did data management for a company that performed a high volume of outbound business dials (not consumer lines). At one point we evaluated productizing our non-valid numbers list, so that businesses could do things like flag when their main contact at an account was no longer at the company, triggering an automatic alert to follow up with the remaining contacts at the company and re-establish a relationship. CNAM lookup services like Twilio Lookup[1] don't do so well at this use case, since companies tend to reserve a full block of phone numbers (always showing as active when doing a CNAM lookup), but when an employee leaves their line will temporarily be de-activated internally until it's re-assigned to a new employee.
