Go to my web site, and look under Modular Cables.
While I realize a lot of guys get by using CAT5 and similar connections, you start messing around with the Icom's microphone connection, and you short out pin 1, you're in for an expensive fix!
The other issue are the modular connectors themselves. They look the same as CAT and telephone RJs, but they're different, and they require a different crimp tool. Crimp a round cable with a flat cable crimper, and you end up crushing the wire, and possibly causing a short.