Capture Live GSM Call Voice Packets

I am looking for an experienced telecom/Android developer who can capture the live voice stream (two-way audio) of an active GSM (cellular) phone call.

Requirements:

Capture the live GSM call voice packets/audio stream.
Access both incoming and outgoing audio during the call.
Provide a technical approach for capturing the live voice stream.
Explain any limitations or device-specific requirements.
Share proof of concept or previous experience with similar work, if available.

Please apply only if you have experience with GSM telephony, mobile voice processing, or low-level telecom/audio systems.