And the DAA, while no doubt helping to protect the revenue stream, was arguably necessary until a process was developed for certifying devices to be attached to the network. Although it was hypocritical to require it for the switched network and not for private lines; but then private lines are relatively rare and can be marked as such. I can imagine fly-by-night modem makers turning out products that would put excessive levels into the line (causing crosstalk), unbalanced to ground (causing hum, noise, and crosstalk), leaking power line AC to the phone line (causing shocks to the telephone repair people), having DC leakage on the line (causing false trouble reports and tripping ringing falsely), and on and on. Some devices might not have these troubles to begin with but would develop them when there were lightning strikes around telephone lines.
jhhaynes at earthlink dot net