between PDA ,loud noise

[showsunday@xxxxxxx (showsunday)]

hello ,every body
 
I test my project base on PJSIP(between real WM 5 PDA), got loud noize,but when i test the project between a real PDA and a Emulator or between a real PDA and eyeBeam softPhone ,it's ok.  why?
 
ps:test PDA ?popod O2 830 ,Mio A701),
    connect method?wifi
    OS:windows XP sp2
  
 
  thanks very much
 
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