Users hear an echo when making a call using Skype for Business either as a published app or published desktop with HDX RealTime Optimization Pack. 

Let’s imagine you are UserA and in a conversation with UserB. Echo is when your voice is retransmitted back to you by User B. The effects of this can be distracting and lead to poor user experience, especially in multiparty calls.

SCENARIO
The most important take away here is that echo is not produced on the side where it is heard.
Example of a conversation with no acoustic echo cancellation (AEC):

• User 1 places an audio call to User 2
• User 1 is using a headset (composite device) as an audio device.
• User 2 is using a separate device for speakerphone and microphone (Non-Composite device) where speakerphone does not have an AEC. (acoustic echo cancellation).
• User 2 does not hear the echo from their end. 
• User 1 hears his own echo when speaking.

There are two types of echo cancellation methods:

1.     Hardware-based acoustic echo cancellation (done by an audio device in use).

• Citrix recommends that you use a headset with integrated speaker (Composite device) or speakerphone that has hardware-based echo cancellation. Where the Echo is canceled by the AEC module on the hardware device.
• Some of the Skype for Business ready devices which can perform AEC in hardware are listed here https://partnersolutions.skypeforbusiness.com/solutionscatalog

2.     Software-based acoustic echo cancellation (done in our case by the Real-Time Optimization Pack Media Engine).

• If the user does NOT have a headset with integrated speaker (Composite device) or external speaker with no inbuilt AEC nodule, The HDX RealTime Optimization Pack AEC engine provides software-based acoustic echo cancellation.  
• Until HDX RealTime Optimization Pack 2.7 the software-based AEC in Real-Time Optimization Pack Media Engine on windows does not produce effective  AEC. (acoustic echo cancellation).

Example of  a conversation with acoustic echo cancellation (AEC) active:

Troubleshoot the Acoustic Echo Cancellation in RealTime Optimization Pack 2.8