The Echo that the other person hears is caused by hardware deficiencies on some android devices, which feeds audio from the speaker to the microphone.
To mitigate this problem Talkatone includes Software Echo Cancellation. However, in order to effectively cancel out the echo, a bit of fine-tuning is required.
Please press Menu button while in Talkatone, then "Settings" and "Call Settings". On that screen you will see "Echo patch delay" parameter. You may need to try different values and find the one which works to eliminate the echo.
Yes, we tested on many devices, however, it's virtually impossible to buy every single device and to make it worse sometimes this parameter may be specific for individual device (e.g. 2 devices of same model would need different setting).
So, you may try to find appropriate value for this parameter which works on your specific device. This parameters needs to be found with 5 (ms) precision in order for echo to be completely eliminated. When you are within 10ms of "sweet spot" the echo will start going on and off during the call - this means just little bit more fine-tuning needed.
Usually this parameter is somewhere between 160ms (on ICS devices) and 350ms (some HTC-made devices and tablets). For your reference, some users have reported that 125ms resulted in high quality audio on Droid Maxx devices.
p.s. if you are able to find the value which works for you, please let us know what it is, so we can push this config change to all users who use the same model.