600 An operation is pending.

601 An invalid port handle was detected.

602 The specified port is already open.

603 The caller's buffer is too small.

604 Incorrect information was specified.

605 The port information cannot be set.

606 The specified port is not connected.

607 An invalid event is detected.
Restart your computer to make sure that all recent configuration changes have taken effect

608 A device was specified that does not exist.

609 The device type was specified that does not exist.

610 An invalid buffer was specified.

611 A route was specified that is not available.

612 A route was specified that is not allocated.

613 An invalid compression was specified.

614 There were insufficient buffers available.

615 The specified port was not found.

616 An asynchronous request is pending.

617 The modem is already disconnecting.
Wait for Network and Dial-up Connections to finish disconnecting.

618 The specified port is not open.

619 The specified port is not connected.

620 No endpoints could be determined.

621 The system could not open the phonebook.

622 The system could not load the phonebook.

623 The system could not find the phonebook entry for this connection.

624 The system could not update the phonebook file.

625 The system found invalid information in the phonebook.

626 A string could not be loaded.

627 A key could not be found.
Recreate connectoid. Uninstall and reinstall DUN.

628 The connection was closed.

629 The connection was closed by the remote computer.

630 The modem was disconnected due to hardware failure.

631 The user disconnected the modem.

632 An incorrect structure size was detected.

633 The modem is already in use or is not configured for dialing out.

634 Your computer could not be registered on the remote network.

635 There was an unknown error.

636 The device attached to the port is not the one expected.

637 A string was detected that could not be converted.

638 The request has timed out.

639 No asynchronous net is available.

640 A error has occurred involving NetBIOS.

641 The server cannot allocate NetBIOS resources needed to support the client.

642 One of your computer's NetBIOS names is already registered on the remote network.

643 A network adapter at the server failed.

644 You will not receive network message popups.

645 There was an internal authentication error.

646 The account is not permitted to log on at this time of day.

647 The account is disabled.

648 The password for this account has expired.

649 The account does not have permission to dial in.
This error can occur if the username is 'system' or the same as domain name for people running networks.

650 The remote access server is not responding.
Error 650 can also occur if the ISP server modem/equipment is not set up properly.

651 The modem has reported an error.

652 There was an unrecognized response from the modem.

653 A macro required by the modem was not found in the device .INF file section.

654 A command or response in the device .INF file section refers to an undefined macro.

655 The [MESSAGE] macro was not found in the device .INF file section.

656 The [DEFAULTOFF] macro in the device .INF file section contains an undefined macro.

657 The device .INF file could not be opened.

658 The device name in the device .INF or media .INI file is too long.

659 The media .INI file refers to an unknown device name.

660 The device .INF file contains no responses for the command.

661 The device .INF file is missing a command.

662 there was an attempt to set a macro not listed in the device .INF file section.

663 The media .INI file refers to an unknown device type.

664 The system has run out of memory.
The system has run out of memory. Close some applications, and redial

665 The modem is not properly configured.

666 The modem is not functioning.

667 The system was unable to read the media .INI file.

668 The connection was terminated.

669 The usage parameter in the media .INI file is invalid.

670 The system was unable to read the section name from the media .INI file.

671 The system was unable to read the device type from the media .INI file.

672 The system was unable to read the device name from the media .INI file.

673 The system was unable to read the usage from the media .INI file.

674 The system was unable to read the maximum connection BPS rate from the media .INI file.

675 The system was unable to read the maximum carrier connection speed from the media.INI file

676 The phone line is busy.

677 A person answered instead of a modem.

678 There was no answer.

679 The system could not detect the carrier.

680 There was no dial tone.

681 The modem reported a general error.

691 Access was denied because the user name and/or password was invalid on the domain
Check the username and password, also check if account is activated.

692 There was a hardware failure in the modem.

695 The state machines are not started.
Uninstall and reinstall dialler, network, and DUN. Send to manufacturer

696 The state machines are already started.
Uninstall and reinstall dialler, network, and DUN. Send to manufacturer.

697 The response looping did not complete.

699 The modem response caused a buffer overflow.

700 The expanded command in the device .INF file is too long.

701 The modem moved to a connection speed not supported by the COM driver.

703 The connection needs information from you, but the application does not allow user interaction.

704 The callback number is invalid.

705 The authorization state is invalid.

707 There was an error related to the X.25 protocol.

708 The account has expired.

709 There was an error changing the password on the domain. The password might have been too short or might have matched a previously used password.

710 Serial overrun errors were detected while communicating with the modem.

711 The Remote Access Service Manager could not start. Additional information is provided in the event log.

712 The two-way port is initializing. Wait a few seconds and redial.
Do exactly as it says, wait a while and reconnect

713 No active ISDN lines are available.

714 No ISDN channels are available to make the call.

715 Too many errors occurred because of poor phone line quality.

716 The remote access service IP configuration is unusable.

717 No IP addresses are available in the static pool of remote access service IP addresses.
This error only happens when the user dials into an NT RAS server

718 The connection timed out waiting for a valid response from the remote computer.

719 The connection was terminated by the remote computer.

720 The connection attempt failed because your computer and the remote computer could not agree on PPP control protocols.

721 The remote computer is not responding.

722 Invalid data was received from the remote computer. This data was ignored.
A PPP packet was received that was not in a valid format

723 The phone number, including prefix and suffix, is too long.

726 The IPX protocol cannot be used for dial-out on more than one modem at a time.
The user should probably not be using IPX to connect. Correct their settings accordingly

728 The system cannot find an IP adapter.

729 SLIP cannot be used unless the IP protocol is installed.

731 The protocol is not configured.
Go to Control Panel/Network and make sure that TCP/IP is installed and configured properly

732 Your computer and the remote computer could not agree on PPP control protocols.

733 Your computer and the remote computer could not agree on PPP control protocols.

734 The PPP link control protocol was terminated.

735 The requested address was rejected by the server.

736 The remote computer terminated the control protocol.

737 Loopback detected.

738 The server did not assign an address.

739 The authentication protocol required by the remote server cannot use the stored password. Redial, entering the password explicitly.

740 An invalid dialing rule was detected.

741 The local computer does not support the required data encryption type.

742 The remote computer does not support the required data encryption type.

743 The remote server requires data encryption.
You should not come across this error as most ISP's servers do not require encryption

751 The callback number contains an invalid character. Only the following charactersare allowed: 0 to 9, T, P, W, (,), -, @, and space.

752 A syntax error was encountered while processing a script.

753 The connection could not be disconnected because it was created by the multi-protocol router.

754 The system could not find the multi-link bundle.

755 The system cannot perform automated dial because this entry has a custom dialer specified.

756 This connection is already being dialed.

757 Remote access services could not be started automatically. Additional information is provided in the event log.

758 Internet connection sharing is already enabled on the connection.

760 An error occurred while routing capabilities were being enabled.

761 An error occurred while Internet connection sharing was being enabled for the connection.

763 Internet connection sharing cannot be enabled. There are two or more LAN connections in addition to the connection to be shared.

764 No smart card reader is installed.

765 Internet connection sharing cannot be enabled. A LAN connection is already configured with the IP address required for automatic IP addressing.

766 The system could not find any certificate.

767
Internet connection sharing cannot be enabled. The LAN connection selected on the private network has more than one IP address configured. Reconfigure the LAN connection with a single IP address before enabling Internet connection sharing.

768 The connection attempt failed because of failure to encrypt data.
Disable encryption in the connection properties.

769 The specified destination is not reachable.

770 The remote machine rejected the connection attempt.

771 The connection attempt failed because the network is busy.

772 The remote computer's network hardware is incompatible with the type of call requested.

773 The connection attempt failed because the destination number has changed.

774 The connection attempt failed because of a temporary failure. Try connecting again.
Reboot. Check dialler settings. Reinstall dialup components.

775 The call was blocked by the remote computer.

776 The call could not be connected because the destination has invoked the Do Not Disturb feature.

777 The connection attempt failed because the modem on the remote computer is out of order.

778 It was not possible to verify the identity of the server.

779 To dial out using this connection, you must use a smart card.

780 An attempted function is not valid for this connection.

781 The encryption attempt failed because no valid certificate was found.

Link for fix's

http://talktalk.opaltelecom.net/html/dun.html