Sip host ip changes

Hi,

I am trying to evaluate a pure sip solution.
I have 2 sip software clients that are working against an Asterisk server.

When i add qualify=yes in sip.conf the server monitors the client and then i noticed that one client becomes UNRECHABLE.
Meaning the client ip have changed and from that point on the other client could not dial him.
Of course that when the client reregisters all if fine and his STATUS becomes OK and not UNRECHABLE.

What is the cause of this problem ?
How can i fix it ?

Thanks,
0L

set the registration time on hes equipment sorter than the time his ip changes, if its remote then set it to 180 seconds, this will also keep any nat sessions live on his router.

Is there any reason to believe that ISPs who do this suppress it when there is activity? If not, there will still be a time between the change and the next registration.

I tend to see it as a deliberate abuse, by ISPs, of the dynamic IP mechanisms to differentiate cheap accounts.