You'll get a different answer depending on who you ask. Ultimately it's up to you.
As a start I would say any general 'standardisation' of feature codes will be driven by the T1 telcos of any given country. For example, 'last-call-return' will probably be *69 if you're in the US, but 1471 in the UK (because that's what BT do). In the UK it's commonly accepted that dialling 141 before your number will withhold your CLID.
WIth a feature that doesn't exist on a T1 network or is at least uncommon, such as dialling a code to have your number played back to you, I'd say it's unlikely there's a 'best practice'.
For your purposes it's also worth considering what requires FreeSWITCH's functionality. For example, Do Not Disturb can be implemented on the phone without having to necessarily any other parties there and then that it has changed state. This is the same with call waiting (depending on if you've restricted concurrent channels on FreeSWITCH or not in order to achieve it).
tl;dr There's no best practice. It all depends on the specific of how you've implemented it!