If you look at how incredibly thin standard telephone wires are, it seems incredible that with the right technology you can transmit up to 100 Mbit/s over a pair of them...
Most telephone interconnects will be at least 24 AWG, some short run cables may be 26 AWG (same as Cat 5). Really old interconnects will often be 22 and even as high as 18. And the thing is, these are usually worse from a high frequency transmission line standpoint as they weren’t designed for anything above 4khz voice.
Poor noise rejection, cross talk, poor loss characteristics, and inconsistent termination are the more challenging issues in pressing high speed out of the phone network.