IAX register string

Hello all

Need some help with the small issue.

With SIP we can register as username:password@host/callbackextension and we will receive incoming calls on “callbackextension”; without “/callbackextension” incoming calls will come to “s”, so “s” should be handled in the dialplan.

But what about the same behavior in IAX?
Currently all my incoming IAX calls are addressed to “s” as I can see it in a iax2 debug:


and my register statement looks like username:password@host

How can I register with my IAX provider with something different from “s”? Is there an IAX equivalent of username:password@host/callbackextension used by SIP?