How well can digital phone lines carry dial-up connections? It was an issue in the early days of digital lines - same as with fax. For fax we got v.34 and other improvements. IDK about dialup tho.
Faxes routinely get 9600 and 14,400 speeds over these ATAs; it helps a great deal if your coded is G711 which is an uncompressed codec (it is u-law PCM in the USA; PCM is just like the bits recorded on a music CD, but at a much lower bitrate).