Asterisk 1.4.0 using asterisk-gui

I have installed asterisk 1.4.0 with the web interface. I added some sip users and iax2 users. I can call the other sip users from both the iax and sip users, but when I try to call a iax2 user it gives me this error:

-- Executing [2003@numberplan-custom-1:1] Macro("SIP/2001-08de9fa0", "stdexten|2003|IAX/2003") in new stack
-- Executing [s@macro-stdexten:1] Dial("SIP/2001-08de9fa0", "IAX/2003|20") in new stack

[Jan 5 18:55:16] WARNING[6475]: channel.c:2910 ast_request: No channel type registered for ‘IAX’
[Jan 5 18:55:16] WARNING[6475]: app_dial.c:1081 dial_exec_full: Unable to create channel of type ‘IAX’ (cause 66 - Channel not implemented)

How can I fix the definition of iax user 2003 from iax to iax2? It appears that the extensions are not stored in the extensions.conf, where are they stored or can they somehow be modified.