Automatic call recording: Storing VOIP calls as mp3

This is a time consuming task, and your box is probably not able to do that in real time.

I'm also searching for a solution to record all incoming and outgoing calls with a FritzBox, but specially isdn gateways.

Today, there is no solution to solve it acceptable for me.

Now, all calls from isdn and voip are routed via an external asterisk server, which can record and store it as a mp3 file. Maybe the asterisk mod for FritzBox can do the same for you? (I don't test it!)

For Voip calls only, you can search for dtmfbox mod, so i think there was a chance to store the voip-call-data on a usb storage (but also untested by myself!)

It would be possible to have a process on the box to dump the raw packets. tcpdump could be used for that. Depending on your bandwith, it may be possible to upload that in realtime to a FTP server. It might be necessary to open the FTP connection in advance. Conversion to MP3 will probably be too much.

tcpdump is using a library named pcap. There is probably a documentation for it, or you can look how tcpdump uses the library.
Or you can use tcpdump itself to dump the raw packets to stdout and feed that into your program.
You must decide whether you want to just dump the packets to some file and do the processing on the server, or do the processing on the box.
The FTP part is quite simple, just look at the RFCs regarding FTP.