This does not mean that a domain with another extension, for example a .com, is not visible on mobile devices and tablets. However, it is more than likely that the developer of a .com website has not designed their site to be viewed on a mobile phone. That is precisely what .mobi domains are for, to guarantee that websites under that extension comply with certain regulations that allow them to be visible on mobile devices. In this way, you can have a .com domain so that they can visit you from a PC, desktop or laptop and the .mobi to show your website designed so that they can visit you from a mobile phone.