I am (unfortunately!) able to re-create your issue, just by creating a brand new app and setting a variable to hold the phone number. Save the app, launch on Android and it's fine, launch on iOS and it doesn't work - just launches a browser attempting to reach 'tel'.
I've tried various workarounds, including putting the 'tel:' into the variable too - doesn't work, although stil works on Android.
Interesting that you have colleagues for whom it works and others for whom it doesn't - and you say they're all on iPhones? If that's the case, it would be useful to try and understand any differences between your versions, both of the hardware, OS version and PowerApps build you're all running.
I confirm the issue with my app as well, running latest iOS 10.3.3. It wasn't happening with previous PowerApps version, it seems a new bug was introduced in 2.0.670.
I am using following OnSelect formulas, to strip numbers from spaces, which tended to help to initiate the iOS dialer (a workaround found somwhere on these forums):
Weirdly enough, some numbers still do work and try turn the dialler, but most take directly to Safari. I think the ones with most obvious phone prefix, like starting with +48 or bracketed prefix work.
It's a pretty nasty issue for apps designed as contact hubs, as mine :-(
Thanks for the explanation and examples. I set mine up just like you did on the BrowseScreen and it worked like a champ. However, when I tried to use the same method on the DetailScreen, I cannot get it to launch the phone number. I tried 2 formats, Launch("tel:" & ThisItem.Primary_x0020_Phone) and Launch("tel:" & DataCardValue4), thinking I had to use the value on the data card. So far nothing I tried works on the DetailForm. What is the correct syntax to make it work on the DetailScreen?