M4B is the native format of Audiobooks on all iOS devices and is famous for its simplicity. If you are looking for how to open an M4B file on your iPhone, then you need to visit the iBooks section of your iPhone as all the M4B files stay there. There you will see all of your audiobooks, just tap on them and listen to those audiobooks. If you don't have any audiobooks, then transfer them using the WALTR PRO application.