Most boot loaders will sit at their prompt forever after receiving a single character of line noise.
Some modems will let the RS-232 signals "float", sending noise when there is no caller. Because the modem is not asserting Data Carrier Detect it expects the receiver to discard the noise characters.
The combination of an unfortunate boot loader with an unfortunate modem can result in a machine that will regularly hang during booting.
If you cannot configure your boot loader to obey DCD then be careful to test any modem you intend to purchase to ensure that it does not generate characters when their is no caller. At the present only SYSLINUX implements full RS-232 status signals.