The Zoiper webphone is using just an npapi browser plugin which has a lot of limitations:

• it is entirely based on two outdated technology (npapi and activeX)
• poor browser support:
  • doesn't work in Android, iOS, MAC OSX, Linux (Windows only)
  • doesn’t work with latest Chrome anymore (Chrome already removed the support for npapi and never supported ActiveX)
  • never worked in Edge

The Mizutech webphone is based on modern HTML5 technologies and has multiple built-in engines which offer a wider coverage (all browsers, all OS, even mobile browsers support: windows, linux, mac, ios, android, Chrome, IE, edge, firefox, safari, opera and others )
The webphone is flexible and customizable: shipped with ready to use html/css which can be freely customized/modified without the need to reinvent the wheel.

	Zoiper	Mizutech
Chrome
Firefox
IE
Edge
Windows
Linux
MAC
Android
iOS

You might find other solutions marketed as "Cross platform" but they are mostly use one single engine and are far to be cross platform. Even if on platform X the performance is not optimal, they might not take full advantage of the platform capabilities as the webphone does by using the best available engine.