That became a common practice before typical modems were able to identify call progress elements like dialtone, ring, busy, etc. Long after that changed, intero

That became a common practice before typical modems were able to identify call progress elements like dialtone, ring, busy, etc. Long after that changed, interoperability problems with the ad-hoc in band signaling conventions meant they were often disabled by default. Couple this with the fact that up through 9.6k or some negotiation problems were audibly diagnosable and the feature itself made sense up to a point. Inertia and industry dynamics probably account for why it persisted into the consumerization of the Internet - by this point the consumer market was dominated by low margin white box manufacturing. Keep in mind that the very high rate of change in signaling standards (9.6 -> 14.4 -> 19.2 -> 28.8 -> 33.6 -> 56 in roughly ten years) seemed to essentially leave little energy for non-core engineering changes.