hi all! i’ve tried all the echo cancellers in zconfig.h and all compiled well, zaptel and asterisk. but still when i’m calling to pstn am having a voice feedback. very undesirable. anyone who’ve configured this echo problem already, need a little help. 
It could be all sorts of things… Where is the echo coming from?
It could be your phone, your cpu (of the * server), can you give some details on those?