SunCom does not support IM on any phones except the HipTop, Blackberry, and Windows Mobile based phones. You can likely find some sort of Java applet somewhere that will let you IM on their other phones, but they do not have the capability built into the phones like T-Mobile does.
Also, just in case you didn't know, IPRelay is available on the HipTop. There is an IPRelay client available for Blackberry also, but last I heard SunCom was having some trouble getting it to work on their system.