3Com Courier, Reference

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V.90 Capabilities 13-3
If you enable the V.8 call indicate tone, expect to hear a sound like a fast
ringing signal while the call is being connected.
V.34 If the remote device has V.34 capability, the Courier V.Everything mode
uses a line probing technique to determine the highest speed possible
under current line conditions, then they complete the connection. If the
remote device does not have V.34 capability, the Courier listens to the
device's answer tones to identify the standard rate at which the remote
device is operating, and then adjusts to that rate.
When the Courier answers a call, it sends out a series of answer tone
signals until both devices negotiate the best connection rate.
V.90 Capabilities
The Courier V.Everything modem has V.90 capabilities. The Courier
V.Everything modem can dial into V.90 servers to establish speeds up to 56 K
downstream.
Other Protocols
x2 x2 client modems can receive data at speeds up to 56 kbps and send data
at V.34 speeds. To use x2, the client x2 modem must connect to a server
x2 modem. If clients attempt to connect to ISPs that do not use x2, the
client modem will negotiate the next available modulation. For example,
an x2 client modem calling into an ISP that only supports V.34, the
modem will only negotiate the highest v.34 connection rate. The
maximum V.34 connection speed is 33.6 kbps.
Fast Class (V.FC)
Handshaking
After trying V.34, the Courier V.Everything modem tries for the fastest
possible V.Fast Class (28.8 Kbps) connection. In order to negotiate V.FC,
V.8 has to be disabled. V.FC is not part of the ITU V.8 training sequence.
If the remote device is not V.FC capable, a connection is made using the
highest compatible modulation scheme (V.32
terbo
, V.32
bis
, and so on,
down to as low as Bell 103, or 300 bps)
For your modem to Command
Enable V.90
ATS58.5=0
(Default)
Disable V.90
ATS58.5=1