Nortel Phone in Asterisk

Hi someone knows why I can not uses 2 nortel’s phone in Asterisk? using unistim.conf and UNIStim protocol

The first one works good
The second does the register, but when I make a call I cannot hear the other extension and when I place a call to this extension goes direct to mailbox.

Any idea about this?

I was running a sniffer in orther to get some differences and I placed a call from a computer with a softphone and WireShark. I got the packages. I saw that with the phone who works the Server and softphone send and receive RTP packages (both directions) and the other case (not working phone) just softphone send RTP packages but Server never answer