While it’s possible to get some great shots with an iPhone or an Android phone, you are limited by the phone’s little plastic lens, which isn’t as good or versatile as the ones you’ll find in many dedicated digital cameras. Phone lens lack optical zoom and often don’t have as wide a view as you’ll find with digital cameras.
There are some options such as Olloclip for iPhone and Photojojo for iPhones and Andorid phones.
For more on both brands, check out Consumer Reports review, “Add-on lenses for smartphone cameras tested by Consumer Reports”