Not exactly the same, but you could implement this in a similar way to the text editing, but with custom handling for your telephone numbers.
You would essentially look at the existing runtime menu function handlers and write a similar one for your purposes. I suggest you take a look at how the existing ones implement the required callback function. Take a look at the text menu item and its callback function:
https://www.thecoderscorner.com/ref-docs/tcmenu/html/class_text_menu_item.html
The callback function is textItemRenderFn implemented in src/RuntimeMenuItem.cpp
At the moment the designer does not support custom classes so you'd have to add them to the menu yourself at runtime (easiest to put them at the end of a menu by pointing the last item's next to this and setting the custom item's next to NULL). You'll see this done in many of the examples.
We'll need to think about how these can be integrated into the designer plugin system, probably by providing an XML definition of how to generate the code for these at some point.