In the FVWM Crystal manager there is a link for sipcrack
however selecting it will only return an error as shown.
 cd: 1: can't cd to /pentest/voip/sipcrack/
-e: 1: ./sipcrack: not found
Sipcrack however is not installed.

You can add it by following the link here or just delete the link itself.

This is in the vmware version of bt4