See page 36 of the PSR-E453/PSR-EW400 Owner's Manual, where it talks about "Playing an External Audio Device with the Built-in Speakers." The illustration shows a standard "auxiliary audio" cable with 1/8" TRS (stereo) plugs on both ends, but some phones have a jack that takes a smaller plug, and some phones take a TRRS plug rather than a TRS plug, so the exact cable you need might vary from the one depicted in the illustration.
By the way, if TRS and TRRS are unknown and mysterious terms, they're actually quite simple. Audio cables can be designed to conduct signals that contain one, two, or three audio channels.
An audio cable that carries only one audio channel (or mono) is a TS cable, which stands for "Tip/Sleeve." It will have a single painted groove that goes around the plug, separating the surface into two areas-- the "tip" (at the very end of the plug) and the "sleeve."
An audio cable that carries two audio channels (usually stereo, but also used for balanced mono) is a TRS cable, which stands for "Tip/Ring/Sleeve." It will have two painted grooves that go around the plug, separating the surface into three areas-- the tip, a "ring" (between the two painted grooves), and the sleeve.
An audio cable that carries three audio channels (usually stereo out for headphones plus mono in for microphone) is a TRRS cable, which stands for "Tip/Ring/Ring/Sleeve." It will have three painted grooves that go around the plug, separating the surface into the tip, two rings, and the sleeve.
There is also a fourth type of plug-- TRRRS-- which has four painted grooves, but it seems to be used very rarely.
So you should check your phone's user guide to see whether the headphones jack takes a TRS plug or a TRRS plug, as well as whether it takes a 1/8" (or 3.5mm) plug or a 3/32" (or 2.5mm) plug.