The port that the modem is using is currently open by another application.

To se desilo samo od sebe tj. prije je modem radio na COM3?

Najjednostavnije bi bilo da modemu u properties-ima postaviš COM4 pa vidi što se tada desi.

http://modemhelp.net/dunerror/error_633.shtml

To change the COM port for a modem
1.
Open Phone and Modem Options in Control Panel.

2.
On the Modems tab, click the modem you want to configure, and then click Properties.

3.
On the Advanced tab, click Advanced Port Settings.

If this option is not displayed, the modem does not support changing the assigned COM port

4.
In Com Port Number, click the port number you want to use.


Notes•
To perform this procedure, you must be a member of the Administrators group on the local computer, or you must have been delegated the appropriate authority. If the computer is joined to a domain, members of the Domain Admins group might be able to perform this procedure. As a security best practice, consider using Run as to perform this procedure.
•
To open a Control Panel item, click Start, click Control Panel, and then double-click the appropriate icon.
•
Do not change this setting unless it is required by your system configuration. The communications port is normally assigned correctly when you install a modem.
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The advanced COM port settings are available only for certain brands of modems. If you do not have one of these modems, to change the assigned COM port, you must reinstall the modem and choose the desired COM port.