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.