SOMETIMES one side dont hear

Hi, i have this configuration:

Asterisk Server on my office
the router has DMZ to the asterisk IP.
Internal IP:
External IP: is changing, so i use no-ip with no ip service runing on the server.


the users have nat=yes and allow=alaw,ulaw

Okey, now the problem:

I have a SIP DEVICE on my house, is behind a router, the internal ip is i configure one extension on it, the registers works okey, but when i make a call to the SIP DEVICE the callee party dont hears the other party (the caller hears ok)

now, somedays works (the conversation is okey), and somedays not…

why it can be?