why the audio voip have no use for AEC?

I use audio unit at iphone 6 ,use to voip,and use the spaker and head mic on the phone.

i set the conponentsubtype = kaudiounitsubtype_voiceprocessingIO;

AND set the property kauvoiceioproperty-bypassvoiceprocessing, kaudioscope_global, 0,0

and the i use audiosession control the device use kaudiosessionmode_videorecording (when i use voicechat,the AEC delay time will so long ,may be more than 3 second )

but the audio unit voip seting is no use for my project .


i am not use headphone,so the audio unit voip is really usefull ?

and how to use it to echo cancellation?