Why are there problems when connecting to the Internet using dial-up connection?

The Operator Assisted or Manual Dial dialog box appears and internet dial-up connection cannot be established.

This symptom occurs when the Operator Assisted Dialing option in the Advanced menu of Network Connections is selected.
To resolve this symptom, diable the Operator Assisted Dialing option and then connect to the Internet via dial-up connection.

Note:
The Operator Assisted Dialing function is sometimes used in communications in deemed audio that communicates through the connection of computers to the earphone/microphone jack of analog type of cellular phones or PHS telephones through a dedicated cable.
If making off-hook dialing on a portable phone, click OK when the Operator Assisted or Manual Dial message appears, and use dial-up connection to connect to the Internet.
The communication speed is 14,400bps maximum (depending on communication states), but cannot be used with the current dominant digital PDC (800MHz/1.5GHz).

* Help of Windows XP has the following description on Operator Assisted Dialing.
Overrides automatic dial-up situations where intervention is required. For example, when making calls through a manually operated switchboard.