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