Some fixes may be app specific but for echo issues that aren't caused by environment they are often a product of the service. People with internet calling often complain of echo because transmission speed or service feedback. It could also be caused when the device on the other end bounces audio back due to high volume levels or poor noise cancelation. The screech could also be feedback. Usually with feedback you move the microphone away from the speaker but since they are stagnate on a phone it could be a case of phone position, volume too high or poor noise cancelation causing reverb. Both issues could be caused by the same condition. Try a Bluetooth ear piece, earbuds or headphones with a mic built in and see if the issue improves.