The Atari ST models have this RS-232 port built in. His latest revision is 3.0, available free from most bulletin boards. But at a speed of 300 baud they are slow by today's standards. At 1200 baud, the SX212 is four times faster than the older Atari modems. The SX212 is Hayes compatible when it operates via the RS-232 interface.
Hayes commands are the standard by which all modems are measured.
I've been using an Anchor Mark XII for nearly 3 years without any problems. It is extremely reliable and quite Hayes compatible. My only complaint about this unit is that it doesn't have a built-in speaker. I like to use a modem speaker to listen to the sound of telephone signal tones. The current crop of Anchor modems, including their new Lightning 2400 baud series, all have speakers.