Android Native: After hang-up unable to call again

Hi all,
I am experiencing a strange problem with the Android Native Sample.
It happens that the first call is correctly accomplished (with audio/video correctly flown) while if I hang-up and then call again it happens that:
- Video start to be seen on the device
- SDP offer is never generated
It seems like the sdk is unable to re-initiate the call.

Do you know how can i face this? Do you need some more detailed information?
Thanks,
m