First off, you have to be careful with recording calls as there are legal requirements depending on the country.
Here is a blog post on how to record audio using the MediaRecorder.
I haven’t tried recording phone call’s but there is a option in MediaRecorder AudioSource for:
- VOICE_CALL – Voice call uplink + downlink audio source
- VOICE_DOWNLINK – Voice call downlink (Rx) audio source
- VOICE_UPLINK – Voice call uplink (Tx) audio source
As long as the audio source options work, you should be good to go.