When I connect a call for testing, echo makes it difficult to have a conversation

When you perform testing, if the caller and callee devices are close to each other and you're using the speakerphone, a feedback loop is created and the echo gets louder. This echo cannot be removed.

In this case, you can prevent echo by muting microphones on both devices.

In general, since the other party on the call is not right next to a user, the internal echo cancellation filter will operate during the service. When testing with the two devices close together, be sure to turn off the speakers or mute the microphones.

Fo more information on how to improve the surrounding environment to prevent echo, see VQE for delivering good audio quality in the blog.