Asterisk looses IAX-registration

When I register with my Zoiper phone to my Asterisk-server, the CLI states this :

asterisk*CLI> iax2 show peers
Name/Username Host Mask Port Status
kristof/kristof (Unspecified) (D) 255.255.255.255 0 UNKNOWN
jonaskellens/jo (Unspecified) (D) 255.255.255.255 0 OK (17 ms)

Then a few seconds later :

asterisk*CLI> iax2 show peers
Name/Username Host Mask Port Status
jonaskellens/jo (Unspecified) (D) 255.255.255.255 0 UNKNOWN

In my iax.conf file, I have put qualify=yes to make sure that Asterisk doens’t loose track of my IAX-client.

But still… I’m registred for a minute or so, and then nobody is able to reach me on my softphone because Asterisk doesn’t know to which IP-address to send the call…

What can I change to my setup ?