Hmm this remembers me a problem I had two years before with Ionic 1. I resolved the problem when I added this lines to my config.xml
<access origin="*" />
<access origin="http://*" />
<access origin="https://*" />
… or simmilar. I had to explicitly enable usage of http and https protocolls (allthough the file protocoll, tel protocoll, any one I used in the app). Perhaps this could help …
You could allthough have a look at this: <access origin="tel:*" launch-external="yes" /> not worked IOS