Giuseppe,
Clearly if you did your capture of normal VOIP conversations, your capture is inadequate. However you have not provided enough information for us to help you. We would need to know
* How you performed the capture? Did you use port-mirroring, or did you use filters (possibly incorrectly).
* What is the network topology (were there multiple interfaces connected to the interface, meaning that your capture tool only saw traffic going on one of the interfaces and one direction?). This could apply to any switch our router in the path, depending on where you did the capture.
