David Nebenzahl wrote:

Everything seems configured correctly: I've set the speed at both ends
to 56K and disabled waiting for dial tone.

So what am I doing wrong here?


The maximun speed you should set for is 33.6 kbps analogue.

Above that, the communication relies on connection to an ISP's modem
rack port supporting digital network signalling, something that is
missing on your back-to-back attempt.

--
Adrian C