Of the roughly 150 mln wireless users in the U.S. last year, only about 10 mln to 20 mln people used prepaid phones. That's somewhere in 6-10% range. But those numbers are rising. Years after their European counterparts, major U.S. carriers have finally taken notice of the market for prepaid customers. At $2.1 bln in 2003, it was becoming difficult to ignore. The prepaid market grew 19% between 2002 and 2003, according to trade group Cellular Telecommunications & Internet Association.