Cellphone MP3 with Chatterbox FRS-X2?

[Slacker]

I've been playing around with using my cellphone's (Motorola E815) MP3 player with my Chatterbox FRS radio.  The problem is that the cellphone won't switch to the 2.5mm jack as output when connected to the Chatterbox audio-in jack.  It keeps playing through the phone speaker.  The phone works fine with earphones or a headset.  I tried jacking it into stereo through the stereo's 'line-in' connection.  Same problem.  

I traded some emails with Motorola support, but after getting through their standard troubleshooting questions (are you sure the cord is good, etc.) they fell back to "we can't gaurantee the function of our products with such devices" and pointed me towards a pair of earphones.

Anyone have any luck with this type of setup?