Would erase content and settings affect phone number? [duplicate]
No, your phone number will not be affected.
Your phone number will not be affected.
But try to see what is occupying the space with "Settings -> General -> Usage.
Back up via iCloud or iTunes then go ahead and delete stuff.
Look for apps that use a lots of space and delete them "Delete App and Delete App again to confirm.
Your messages can contain a lots of photos send to you that consume space.
If you are unsure if you want to keep some you need to "Manually Find & Save Photo Messages".