If you so seriously doubt in it, then try to supply your modem by DC. You will break your modem or port.With all the technology around, I seriously doubt that the ac would be used to produce RS232 levels for the comport in a modem besides powering it.
Any doubt?It is a little known fact that some of these modems need an AC 12 to 15 Volts supply at around 250 mA. Supplying them with DC will lead to disaster since the rectifier circuits develop both +ve and -ve voltage rails for the chips and the RS-232 interface. Those circuits need an AC supply to do this.