fax problem

Hi

I have an problem with trixbox, and I appreciate if anyone could help me.

I am tring to configure trixbox to send and receive fax's, but i don't have success.

In the extension configuration, I have the following configuration for fax:
canreinvite = yes
fax extension = system
fax email = mail@mail.com
fax detection type = nvfax

The Re-invite message is not send, what could be the problem for this?

Is necessary install any more software to could send and receive fax's? or is sufficient configure the parameters in the Fax Handling options of the trixbox?

Anyone could help me with this problem?

Thanks a lot