GSM shield voice record with ISD1700, adaption problem

Hello

I want to record GSM voice calls with the ISD1700 voice shield. I thought, it will be no problem, but the recorded voice is really bad. Recording through the microphone is much better. I looked since some days in the datasheet and tested a lot, but I have still "klack, klack" on my recorded voice and it is still too silent.

I tested a lot of connections. Because of the automatic gain control, I would prefer to connect the GSM board audio output to the microphone input of the ISD1700. But this was a worse audio quality than the connection to the line in of the ISD1700.

Has anybody already done it, and knows how to connect both devices?

Thank you in advance.

FelixG