This is a common problem. If you set the amplification too high on the ATA and have a bad or old cable between the ATA and your phone, you can get interference between the gain for the input and the output.
Also, if there's any point along the audio path for the call that doesn't take well to over compensated volumes, such as a switch at the destination end, then you might get echoes there as well.
It's safer to just use the amplification that is built into your phone rather than using the ATA's amplification feature.
Just my $.02.
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