Do you know where I could find a VoIP application that's compatible with both desktop and smart phones?

I really like Zoiper, but I want to caution you about using a softphone on a smartphone. Your SIP login information is vulnerable and can be transmitted in plain text over the network. Trust me, I learned this the hard way.

One of the most popular option is Skype, which is available on Windows, macOS, iOS, and Android. You can easily download it from their respective app stores or from the oficial website.

Another great choice is Zoom, which has become widely used for video conferencing and calls. It also has a desktop version and mobile app that are quite user-friendly. You can sign up for free and download it from the Zoom website or any app store on your device.

If you are looking for something with more features, you might consider Discord. Initially made for gamers, it also provide voice call services and has desktop applications for both Windows and macOS. It is also available on iOS and Android.

WhatsApp is another popular option, especially for personal use. It has voice call option that can be accessed via its mobile app and also from the desktop version. Just keep in mind that you'll need your phone connected to use it on desktop.

For business purpose, RingCentral is a great VoIP solution that provides both desktop and mobile applications, but it may require subscription fee.
Make sure to check their official websites for download link and installation guide. Just remember that these applications may have different features and limitations on each platform, so it would be a good idea to explore which one meets your needs the best.