iax2 registering problem

I am trying to develop an IAX sophone for Android. By now I am doing some tests using libraries with java implementations of the iax2 protocol.
The thing is that when I try to register an IAX2 extension with my Asterisk server I get no success.
I’ve tried differents softphones and the only one who can register is Zoiper, with others (voixphone, yate, …) I always get an REGREJ from the server (the same answer that I get with the java libraries).

Does anyone Know why I only can register with Zoiper? how can resolve this problem?

Thanks a lot!

Do you have In your IAX.conf if you use Asterisk >1.6:

And in your peer configuration requirecalltoken=no