Hear the voice but are not heard

Hi,
i’ve a Asterisk 1.6.1 PBX configured wdth Freepbx 2.5.1.5 online from 4 month. I’ve a isdn (italy) connected to Patton SN4552, it trasmit call to asterisk in ulaw.
From some days when I do some calls or receive it, the audio is only received or transmitted:

  • A few phone calls I hear the voice but are not heard.
  • Other times not feel anything but I feel.
    I have not made any changes to the software in this days. What could it be? A bug?

Thank you.