I believe that the first two paragraphs of WHBM's response were actually about the (non) difference betwen 220 and 240 volts, but he then seems to have strayed a bit off topic.

Modems are worldwide (I assume you mean a modem card in your PC) and nothing to do with electricity voltage. but you may need to understand the difference between analogue and digital phone lines (both in use everywhere in the world) to ensure that you can use the modem wherever you want to. If in doubt, post for advice. Analogue is the traditional standard; fax lines are always analogue. However you can sometimes get difficulties in offices etc where they have installed digital-only phone extensions.