In order to use a modem on the Atari, a modem software handler, or R: device
handler, must be loaded into memory.

There are several families of R: handlers, corresponding to the different ways
in which a modem may be attached to the Atari.

Except for family #7 below, these handlers are used in one of two ways. 
Either they are (A) loaded into memory from DOS just before running the main
terminal application, or (B) the terminal program is appended to the handler,
so that in practice, a single file is loaded from DOS which contains both the
R: device handler and the application itself.

1) 835/1030/XM301 modems.  Atari-only modems, interface via SIO

2) MPP/Supra modems.  Atari-only modems, interface via joystick port

3) SX212 modem/R-Verter Serial Bus Modem Adapter.  
Standard Hayes-type RS-232 modems, interface via SIO

The R-Verter was distributed with four different R: handler versions:

RHAND1.OBJ  - R: handler supporting DSR & RD
RHAND1C.OBJ - R: handler supporting DSR & CD
RHAND2.OBJ  - R: handler supporting DSR & RD and translation tables
RHAND2C.OBJ - R: handler supporting DSR & CD and translation tables

4) SWP ATR8000 interface. Standard Hayes-type RS-232 modems via this interface.

Richard Anderson writes (Oct 2 02): 
  Mine originally came with a driver program; and, I believe, a BASIC program
  to set up the driver from BASIC.  Later they shipped with a special version
  of MyDOS with the R: handler built in.

5) 850 Interface/P:R: Connection, internal

This type of "mini handler" simply loads the R: device handler code from a ROM
chip inside the RS-232 serial interface.  A long beep is heard through the
speaker when the handler is loaded into the computer's RAM.

Many varieties of DOS for the Atari include an explicit provision for loading
this type of R: handler into memory from the 850 or compatible interface.

Also, this type of R: handler is automatically loaded when any 8-bit Atari
computer is turned on with a P:R: Connection or powered 850 connected, but no
powered disk drive is present.

6) 850 Interface/P:R: Connection, external

Used with the 850/P:R: Connection in place of these interfaces' built-in
handlers.

The P:R: Connection was distributed with such a handler, called: PRC.SYS

7) MIO/Black Box interfaces, internal 

These interfaces utilize the PBI or ECI parallel ports on the Atari.  They
include their own R: handlers in ROM, using no computer RAM at all.

8) MIO/Black Box interfaces, external: Len Spencer's Hyperspeed

This handler is "optional" for the Black Box, but "essential" for the MIO
in order to take full advantage of the high-speed hardware handshaking
capabilities of these two interfaces.

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