3com MSR 20-20 참조 매뉴얼

With both references configured, the device always attempts to place return calls 
in the user reference approach and in case failure occurs the dial number reference 
approach. If the command is configured with neither keyword, it equals the dialer 
callback-center user dial-number command.

ppp callback in PPP Commands in Access Volume.

# Specify the device as the PPP callback server, and set the callback reference to 
user mode.

<Sysname> system-view

[Sysname] interface serial2/0

[Sysname-Serial2/0] ppp callback server

[Sysname-Serial2/0] dialer callback-center user

[Sysname-Serial2/0] dialer route ip 1.1.1.2 user Sysnameb 8810052 

# Specify the device as the PPP callback server, and set the callback reference to 
dial number mode.

<Sysname> system-view

[Sysname] local-user usera

[Sysname-luser-usera] password simple usera

[Sysname-luser-usera] service-type ppp

[Sysname-luser-usera] service-type ppp callback-number 8810048

[Sysname-luser-usera] quit

[Sysname] interface serial 2/0

[Sysname-Serial2/0] ppp callback server

[Sysname-Serial2/0] dialer callback-center dial-number 

dialer call-in remote-number [ callback ]

undo dialer call-in remote-number [ callback ]

Dial interface (physical or dialer) view

remote-number: Used for matching ISDN calling numbers. The asterisks (*) 
represent any characters.

callback: Enables the device to place a return call if an incoming number matches 
the number specified by the remote-number argument.

Use the dialer call-in command to configure a valid ISDN calling number and 
with the callback keyword to enable callback for the number.

Use the undo dialer call-in command to remove the setting.

By default, ISDN callback is disabled.