I use Voxi as my mobile provider. Interestingly enough, they don’t have a native mobile app for support. Instead they use a webapp that you can pin to your homescreen (atleast on iOS).
After a bit of googling, and digging through the page-source for their main website, I learnt about this feature in more detail. With a bit of help from iconifier.net I managed to add similar support for this website. Go on.. give it a try.