I had this same problem. So I tried Ivan's suggestion to "use different combinations of the settings in the Audio sections from Zoiper's Settings menu."
For me, these audio settings seem to work:
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Echo cancellation: Hardware
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Automatic Gain Control: OFF
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Noise suppression software: ON
Turning off the gain control made the biggest difference.
For quickly testing different settings, I used my voip provider's test number to say a few words and hear them played back to me.