[Asterisk-Users] asterisk can't find address host. Problem in chan_sip.c

[makevuy]

hello

I have an asterisk server with a public IP address and a nat address
like alias. I have 20 sip clients with private IP address. I don't Know
why, sometimes, when I try to call between 2 phones, y see the next
menssage in the astersk console:

 Can't find address for host 'XX'

What could happen??

Thaks for all and regards