I'm trying to initiate a call on the iPhone with the tel url that has a * in it. It properly brings up the call dialog but drops back to safari when you click call.
<a href="tel:123*12">Test</a>
This documentation from Apple should be helpful:
To prevent users from maliciously redirecting phone calls or changing the behavior of a phone or account, the Phone application supports most, but not all, of the special characters in the tel scheme. Specifically, if a URL contains the * or # characters, the Phone application does not attempt to dial the corresponding phone number.
UPDATE (Jan 2, 2018): The information mentioned here may be outdated. Please refer to new documentation if Apple has relaxed these rules in their newer SDKs. Refer to Husam's answer.