In this case we need to establish your understanding that this issue is not occurring due to anything related to our application. This is a renowned issue on Android devices and all monitoring apps face this issue. So even if you try any other app, you will face this issue as it is related to the firmware of your target phone.
The issue basically occurs due to the firmware settings of the target device. The firmware disables the mic APIs when the target picks up a call, thus causing the recording to be one sided only.
In this case, we cannot guarantee which device will work, as every Smartphone provider has custom firmware settings from the manufacturer according to demand.
However, we can offer you an extension of 20% in order to compensate you for the value. I hope you will understand the scenario.