When I started in the biz in the 60’s, the max was 300. We’d put the phone receiver into a cradle called an acoustic coupler, and the computers would talk to each other over the phone. Ma Bell improved their technology to allow for 600, then 1200, and the race was on. In 1982 I did a project with a Mexican company, but all their phone system could reliable handle was 300.
As we say in the I.T. biz, RTFM.