"You need to convert the logic levels of the uC to the RS232 levels the modem expects."

That depends on exactly what Vikks actually means by "modem"

* If he's talking about a boxed, external modem, as might be connected to a PC's COM port, then you are correct;

* If he's talking about an internal modem card which plugs into a PC's ISA, PCI, PCMCIA or some such, then it will not be RS232;

* If it's a USB modem, it's not RS232;

* If it's a "bare" modem chip (or chipset) it will probably just use straight logic levels.

In other words, as Ijaz has already said, the question is far too vague!