I used to work at an ISO in 1997 and we tested Mototola Sufr 28.8 and USRobotics Sportster 33.6 modems this way, just a short pair between them back-to-back. ATX1D on one side, ATA on the other, connection established.
However, recently a friend of mine tried to use the same trick with two USB modems from Trendnet and it failed. It seems they either don't have oomph enough or they rely on 9vdc off-hook carrier voltage to work.
However, recently a friend of mine tried to use the same trick with two USB modems from Trendnet and it failed. It seems they either don't have oomph enough or they rely on 9vdc off-hook carrier voltage to work.