When managing your data in ArcGIS Online, there is a ca[ability to set a field type which applies a style to the values of a field. There is a glaring omission here.
Why is there no phone number field type? As stated in the documentation, the entire premise for this capability is for better handling of field values as appropriate to their type context:
Define field value types
To help users and client apps identify how field values should be used or represented, you can define what types of values are stored in each attribute field in your hosted feature layer. ArcGIS Online uses field value types to show you the most relevant options when drawing a layer in Map Viewer or configuring popups.
A Phone Number field type would be immensely helpful on client apps as the tel protocol, which would allow client apps to handle the value as a telephone number, automatically passing it to their phone app raising a dialog asking the user whether or not they want to call the number when clicked.
The same thing goes for a URL field type.
Yes. All of this can be handled in your popup configuration but you shouldn't have to. This seems to be the very purpose of setting the field type in the first place.