How do I increase audio sound on my app for recording phone calls?

I have an Echo phone and downloaded an app for recording phone calls, both received and made myself. When it records, my voice is very loud, but you can barely hear the other line unless it is on speaker phone. Is there a way of increasing there level without using speaker? I record phone interviews I do and it is much easier just using the headset and not using the speakerphone...cuts out parts of our conversation.