First you need to make sure that your echo canceller is turned on, this is done in zapata.conf. set echocancel=yes, and echotraining=yes.
You can also try aggressive suppression or try another echo canceller. This involves going to the zaptel source folder and editing (if i recall correctly) zconfig.h. You can define one of several echo canceller routines (ONLY one) and if you are using mark2 you can also define aggressive echo cancellation below. That adds a longer ‘tail’, uses more CPU but i’ve found it helpful when dealing with annoying echoes. After changing any of these settings, you must recompile and reinstall Zaptel. To do this- shut down asterisk and unload zaptel. then do make ; make install. it will compile and install. reload zaptel and start *.
zttest and fxotune are two different things.
zttest tests the accuracy of the timing interface with IRQ interrupts. If this fails you will have problems with conferencing and MOH.
fxotune calibrates the port voltages for your line. This makes the port adapt itself to the line to use the right gains, except it is automatic so it si far more accurate than rxgain/txgain. This probably will help you.
Hope that helps!