The reason I would agree with a fine is not because a caller's identity cannot be unmasked, but because phone numbers are so closely connected with identity that robocallers are essentially engaging in identity theft and slander when they use people's legitimate numbers to spoof their calls.
I had someone call me once, claiming I had just called them. I hadn't of course, so the only thing I could think of was that someone had used my phone number for spoofing.
