Some third-party modems were marketed for use with the Atari 8-bit
computers:

==> Microconnection, by The Microperipheral Corporation
300 bps, Bell 103 compatible, T-SMART software, pulse dialing (not touch tone)
Four versions:
buss-decoding version does not require 850 Interface or equivalent, includes
    DB25 parallel printer interface, with or without autodial
Plain version requires 850 Interface or equivalent, with or without autodial
    and autoanswer

==> MPP-1000C, by Microbits Peripheral Products
300 baud, joystick port 2, Smart Terminal cartridge

==> MPP-1000E, by Microbits Peripheral Products
300 baud, joystick port 2, Smart Term software
 From: "Steven J Tucker" Sun, 13 Jan 2002 16:14:38 -0500
 The 1000E..had this strange problem in that it could never hang up the phone

==> MPP-1200A, by Microbits Peripheral Products
Released? Vaporware?
1200 bps, joystick port 2

==> 300 AT, by Supra (same as MPP-1000E)
300 baud, joystick port 2, Smart Term software

==> 1200 AT, by Supra
1200 baud, Hayes compatible, connects to SIO via SupraVerter/R-Verter cable,
Smart Terminal software

==> Volksmodem, by Anchor Automation
300 baud, 'F' Cable permits connection to joystick port 2

==> Q-MODEM, by Quantum Microsystems
300 baud, two SIO connectors, QuanTerm disk or cartridge

Beyond the above modem models, most any "industry-standard" external serial
modem can work well with the Atari.  These have been commonly sold for PCs for
many years.  The Hayes Smartmodem more or less defined the market for these,
initially.

One common way to use an industry standard external serial modem with the
Atari is to connect it to the SIO port via an Advanced Interface Devices
(A.I.D., later Supra) R-Verter Serial Bus Modem Adapter cable, or
equivalent.

The other common way to use an industry standard external serial modem with
the Atari is to attach it through the 9-pin RS-232-C serial port of the Atari
850 Interface Module or equivalent (such as the ICD P:R: Connection).  One
gotcha here is that the serial port on the 850 is DB9 female, where the PC
world ended up standardizing on a DB9 male connector for this purpose.  But
gender converters are readily available.

For using modems at speeds of 2400 bps and up with the Atari, it will be
useful to have an understanding of data flow control.  Here is a definition
of flow control from www.modems.com:

 Often, one modem in a connection is capable of sending data much faster than
 the other can receive.  Flow control allows the receiving modem to tell the
 other to pause while it catches up.  Flow control exists as either software,
 or XON/XOFF, flow control, or hardware (RTS/CTS) flow control.  With software
 flow control, when a modem needs to tell the other to pause, it sends a
 certain character, usually Control-S.  When it is ready to resume, it sends a
 different character, such as Control-Q.  Software flow control's only
 advantage is that it can use a serial cable with only three wires.  Since
 software flow control regulates transmissions by sending certain characters,
 line noise could generate the character commanding a pause, thus hanging the
 transfer until the proper character (such as Control-Q) is sent.  Also,
 binary files must never be sent using software flow control, as binary files
 can contain the control characters.  Hardware, or RTS/CTS, flow control uses
 wires in the modem cable or, in the case of internal modems, hardware in the
 modem. This is faster and much more reliable than software flow control.

Some 2400 bps modems, and probably all modems with 9600 bps speed capabilities
and up, normally use V.42 standard error correction and V.42bis standard data
compression.  But V.42 requires either software or hardware flow control, and
V.42bis requires hardware flow control (and V.42 error correction).

Hardware flow control is not available with the Atari 850 serial ports.

As a result, just before dialing out with your Atari telecom software, it's
usually desirable, if not necessary, to disable your modem's flow control.

The Hayes modem command to disable flow control looks like:
     AT&K0

The top speed of the Atari 850 serial ports is 9600 bps.

Clay Halliwell offers a tip on utilizing 9600 bps through the 850 Interface:
 On 11 Feb 1996, Marc G. Frank said:

 > I'm having problems getting a modem attached to my Atari 850 to
 > communicate at 9600 baud.  When I set my communications program to 2400
 > baud, everything works fine.  However, when I set it to 9600 baud, the
 > modem echoes my characters but doesn't act on them.  That is, at 2400,

 The problem with the 850 is that some of them (like mine) don't produce a
 PERFECT 9600 baud signal.  As a result modems can't train on it, and while
 they will echo characters back, for some nitpicky reason they won't pick up
 on the "AT" attention code.

 The solution is to do all your dialing at 2400 baud, but set the S37
 register to force the modem to try to connect at 9600.  Then switch your
 Atari to 9600 after connecting.

Through the use of an ICD MIO or a CSS Black Box, it is possible to utilize
modems at speeds up to 14.4 Kbps (V.32bis) at full speed with no loss of data.
The serial R: device handler for the Black Box supports hardware flow control
natively.  Optional for the Black Box, but essential for the MIO, is the
HyperSpeed handler by Len Spencer.

Hyperspd.arc is available at:
http://www.lenardspencer.com/Lenspencer/hyperspd.htm

Modern external modems designed for "modern" PCs now normally utilize a USB
connector rather than the older standard DB9 RS-232-C serial connector.