Telephone call sound is noisy with GSM modem based on SIM 900A

I am using Arduino UNO and SIM 900A GSM modem without shield for receiving telephone calls . Since there is no line input ports on the modem board, I have directly soldered to the line input pins on the Sim ic. I used shielded audio cable but the Arduino board is not shielded
This basic system is working flawlessly but the sound quality is bad and too noisy. Any guide in the right direction?

Did you match the impedance if each connection?
Paul

The input line level audio is coming is directly connected to the SIM pins. The impedance is not matched in this case I supposed, balanced to unbalanced should be correct?. Is it wrong then?

I do not see any spec on the line in pins. What do you have? Historically "line in/out" was 600 ohms. Low impedance. What are you connecting the line to?
Paul

The line input is 600ohms, but I am not sure about the other side- line input impedance of the sim900a pins.

I can't find anything about it either, so I guess it is also low impedance. Can you monitor the audio on the line and see if it is ok at the source? I wonder if the un-connected mike terminals are picking up the noise?
Paul

Source audio is good. When I make call from my phone, the Arduino picks up call and connects the recorded audio through the Sim. The audio is audiable but much distorted. It could be unbalanced input at sim pins, micro level audio input also available, but I have no info.

Possible overdriving the audio. Have you tried reducing the input level from the source? Try putting a 10k pot in series with one of the line input wires and adjust the resistance while listening.
Paul