Phone storage can also be used by photographs, music and a variety of other things. Depending on what phone you have and what version of Android, the Storage option in Settings may give you a breakdown of what is using the space on your phone.
If the problem is things like Media files, you could try installing a file manager (there are many free on Play) and using it to move photos, videos, MP3 etc to the SD card.
If that doesn't help, then you could try:
- clearing the cache on all your apps
- uninstalling apps you no longer use
- uninstalling updates for apps you don't use but can't uninstall
