You can use this to even control two cameras at a time! Just make a second cable and there you go, control two cameras :D
Step 1: Well, collect all the parts needed
*One RJ11 phone line cable
*one switch, two push buttons
*one phone line RJ11/RJ12 splitter
*one 2.5 mm TRRS connector
*one CANON EOS 450D
*cutter and the usual other tools
Step 2: Connecting the wires
#1 will online be connected for FOCUS to the FOCUS switch
#2 will be connected to SHUTTER RELEASE and LONG EXPOSURE SWITCH
#4 will be connected to SHUTTER RELESE, LONG EXPOSURE and FOCUS SWITCH (GROUND)
Your phone line cable has 4 colors.
TRRS connector | RJ11 phone line cable
#1, black color | black color
#2, green color | green color
#4, copper wire | yellow color
part #3 of your TRRS connector and RJ11 red color wire will not be used.
If you have other colors or you want to be sure you have the right wires out of your TRRS connector use a digital multimeter to test it.
Put your digital multimeter on (usually a volume sign or a small speaker or something similar) and touch the red wire with the black wire of your multimeter. It will produce a beep sound that the two wires are connected.
So now put one probe (black) on one part of your TRRS connector. Put the other probe (red) on a wire that sticks out from the behind. If it beeps, it means you have found a pair, otherwise just put the probe on another part or wire until you find a match.