The latest iMac G5 doesn't include a built-in modem for use over telephone lines. Apple has offered a built-in modem on the iMac since the first model was released.
Perhaps the logic is that if you can afford a PowerPC G5-based computer, you probably also have broadband. Apple saves only a few dollars in hard costs, which can multiply into $20 to $40 at the retail price. This cut allows Apple to either keep more profit or shift the dollars to other features on the computer without affecting profit.
For those who need dial-up Internet access or fax services, Apple offers the Apple USB Modem as a $50 accessory, but it appears that you can only order this modem as a build-to-order option. The Apple USB Modem is barely mentioned on Apple's Web site, and you have to go to the Apple Store, select a G5 iMac, and then choose the optional modem to even reach the full specifications.
Those specs say it's a V.92 modem with support for several nifty modern modem features, including caller ID (while online), telephone answering, and modem on hold. These latter two features allow you - with certain extra telephone line features turned on - to have your dial-up modem cake and eat it, too, by answering incoming calls while pausing but not breaking the connection with your ISP. The ISP must have similarly capable devices on their end. The speed of a V.92 modem is a maximum of 56 Kbps downstream and 48 Kbps upstream. It's a tiny, one-piece device that looks more like a USB flash drive than the modems of yesteryear.
I poked around to see what options are available for buying an external modem, as I can't even recall my last such purchase - maybe 1996 or 1997. Small Dog Electronics carries a Mac-compatible Best Data 56K with V.92 support that uses USB and handles Mac OS 8 and later. A quick look at other sites doesn't find better deals.
Reader Robert Pyle wrote to say that he had purchased the Best Data 56K modem and had to tweak a modem script because it did not come with any modem files compatible with Mac OS X. He later wrote back that he found an easier solution: the modem chip in the Best Data comes from Conexant, which also powers the $100 USB modem from Zoom that's Mac-compatible. Download the Zoom Universal CCL scripts, install them, and the Best Data modem will work at its highest rates with a Zoom Universal (115K) modem script selected. The Zoom 2986 comes with genuine technical support, which might make it worth the $50 premium. It's also more readily available from online retailers.