Follow the instructions below and you can't go wrong. It worked for me.
I found this informative list of instructions somewhere on another web site but I don't remember where. My thanks and appreciation to the original author.
Setting up i600 as a modem for your laptop
You need to first purchase the "USB Sync Cable" from either Verizon or Samsung for $60 (or, to save a few bucks, you could use the somewhat bulkier docking station that came with the i600). The sync cable appears to be the same one as used for the Samsung i700 PocketPC Phone edition device, also sold by Verizon (apparently, a lot of the accessories for the i700 will also work with the i600).
*** DO NOT plug in your i600 into your notebook PC using the USB Sync cable until told to do so below. Connecting it too early may prevent the USB modem driver from being installed properly.
1) Make sure your i600 is not docked or otherwise connected to your PC (via the USB sync cable)
2) Stop ActiveSync from using the USB ports via "Connection Settings" in ActiveSync by unchecking the "USB..." option.
3) Insert the "i600 Software Companion Disk" (that has outlook 2000 and activesync 3.7 on it) that came with the i600 into your PC. Cancel out of of the autorun startup app. (The root of this disk contains the "USB Modem" drivers that are needed below)
4) On your i600, go into "Settings", then "More", then "Phone Settings", then "Other". Select "External Data Call" for the USB option.
5) Connect your i600 to the PC by either docking it, or connecting it up with the USB Sync cable.
6) Windows will find new hardware ("USB Modem") and prompt for it's driver. Point the driver installer to the CD-ROM drive containing the disk dicussed above. It will then install the Samsung USB modem and it can then be seen under "Modems" in the Hardware Manager.
7) Create a new dial-up connection with this info:
For Express Network up to 144kbps (must add a 'national access' option to your plan):
Number to Dial: #777
For Quick2Net up to 14.4kbps (might need "mobile web" option added to your plan):
Number to Dial: #7772
A) After installing the above USB Modem driver for the i600, you can then use your i600 as a normal modem for making regular data calls with programs such as PCAnywhere, etc. I do not know the maximum connection speed of the built-in modem of the i600 - my guess is 14.4k. You may even be able to make and receive Fax calls (by setting the Data/Fax mode in "Data Connections" of the "Settings" screen on the i600), but I have not tried this yet.
B) When using the i600 as a modem, I was unable to see any visual indication on the i600's screen that it was in use. Normally, the antenna icon in the top right-hand corner would change to a left and right arrow icon when connected the Internet using the built-in Internet Explorer. But, when I connected my notebook to the net using the i600, the icon stayed as an antenna icon. So, if you don't pay attention, you may not realize the phone is off-hook and using up your minutes.
C) Even though the manual states that an icon with two bar graphs should appear when in data mode (to indicate send/receive activity), no such icon appeared either when connected to the Internet (via #777) or a regular PCAnywhere data call.
D) The "Modem Link" feature that is mentioned in the manual seems to only work for the connection "IrCOMM", which would allow your PC/Pocket PC to use the modem in the i600 without a USB cable (if your PC/Pocket PC has an IR Port). The "USB" option generates an error if you try to activate it, and the "Serial COM1:" won't generate an error, but I don't think there is a serial cable available for the i600 to use that port anyway.