Nice article in the BackTrack Wiki that explains how to sniff phone calls made with DECT devices.
The text explains how to install and run Dedected on BT5. I understand they use Audacity to merge all the WAV files generated by Dedected in one single file, but perhaps there are more tools for this purpose.
UPDATE: I have also found the following video that explains the whole process
Sniffing DECT phones with BackTrack from smtx on Vimeo.