There are plenty of reasons to record your calls and plenty of reasons to be cautious—recording phone calls can be considered illegal in some areas. This Pi project is designed to record calls by dialing into the conversation as a third party.
How it works
As far as hardware is concerned, all you need is a Raspberry Pi, a 3G/GSM adapter, and a SIM card. Alwye was nice enough to share the project code—if you'd like to see how it works, visit the Standalone Call Recorder page on Github. To record a call, you need to first establish a call with the Raspberry Pi using the 3G dongle. Merging this call with a third party will allow you to record the audio from the conversation.
You can read more about the project on the original Reddit post. There was a follow-up thread a few days later with more details. This is definitely a cool concept in our book and a great showcase of what you can do with a Raspberry Pi.