Transmission power hasn't been the cap fo a long time.
The analogue phone line signal is digitised at the exchange, and the digital channel is explicitly 64kbit/s.
Modems can only do "56k" by co-operating with the digital system; one of the modems actually has a digital ISDN connection.
Without that ISDN connection, analogue modems reach up to 33.6k. Theoretically they can do more (but never more than 64k), but in practice that was the last standard produced prior to the 56k, semi-analogue-semi-digital standard.
