Has anyone used the IAX24 J library for incoming calls?

Question

I'm trying to implement an IAX-based voip application for my Android and use the AndroVOIP open source library to make and receive incoming calls, but there is no listener for incoming calls in AndroVOIP, although my registration for IAX was successful. I tried using the Japanese IAX24J client, which is built for NJIAX, but still cannot receive incoming calls.

https://github.com/kmamykin/iax24j/

https://github.com/russellb/androvoip

I want to know how Zoiper and the IAXAgent beta from Playstore make incoming calls. I am looking for a good library project to implement it in my application.