From my understanding, phone add-ons augment the phone’s capabilities by adding hardware not otherwise included in the phone’s functionality. In some ways it seems like accessory could describe the same thing, although the word accessory immediately conjures up purely aesthetic purposes like a cell-phone charm. Accessory does go beyond just something aesthetic. The most I’ve heard the term accessory in regards to phones is when you plug your phone into some non-apple brand dock or use any off-brand cord. It will say “Please unlock phone to use accessory.” This implies that accessory means something that connects your phone to a different object. Like if your phone is going to play music and you connect to a speaker. On one hand, it’s similar to an add-on which also uses the phone as a way to power another functional thing beyond the ability of the iPhone, but in some ways accessory feels like it exists on a larger scale or even that the phone itself becomes the accessory, just as a way to power the ultimate item, like the speakers. Add-on feels more like it would more appropriately be called accessory, based on the way accessory is used in fashion terms. To use a purse as an analogy, an add-on would be a flashlight keychain and an accessory would be a wallet.