As Kevin points out, URL Schemes are the only way to communicate between apps. So, no, it's not possible to launch arbitrary apps.
But it is possible to launch any app that registers a URL Scheme, whether it's Apple's, yours, or another developer's. The docs are here:
Defining a Custom URL Scheme for Your App
As for launching the phone, looks like your tel:
link needs to have least three digits before the phone will launch. So you can't just drop into the app without dialing a number.
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