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UPDATE (Jan 2, 2018): The information mentioned here may be outdated.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.