At the moment you can only enter numbers, '+' and '#' in the phone number fields. It would be much easier to read numbers if all characters are permitted, not just "phone active" characters. Any other characters than ones that are relevant to the phone can simply be ignored. This would allow you to use '(',')', '-' and spaces, in phone numbers, but also let you add text-tags to the phone number. For example, you would be able to enter a US phone number as:
+1 (555) 123-4567
and you can even add things like:
+1 (555) 123-4567 (Old number)
+1 (555) 123-4568 (Current number)
+1 (555) 123-4569 (Parents)
Or whatever other comment you would like.
Interestingly, the contact and phone apps already allow free-form numbers: I imported a set of vcards from my old phone with comments like this and they work excellently (including retaining the free-form text). However, the phone number entry fields simply don't accept other characters than the telephone characters so I can't enter new numbers with comments. So all this requires is to remove the limiter from the phone number text entry field, and I would be happy!
BTW, the phone number entry field also prevents paste if the clipboard contents has other characters than numbers + and #. Unfortunately it does so silently. It took me a good while before I realized why I couldn't copy-paste a number from an SMS into the phone app (the number had spaces in it).