One way to use iCloud to increase space available on the phone is to use iCloud Photo Library beta FAQ - Apple Support but you do NOT delete the photos on the phone in order to gain space. Note the quote below from that link:
If you turn on Optimize [device] Storage, iCloud Photo Library will automatically manage the size of your library on your iOS device, so you can make the most of your device's storage and access more photos than ever. iCloud Photo Library stores the original, high-resolution photos and videos in iCloud and can keep lightweight, device-optimized versions on each of your devices. As long as you have enough storage, recent photos and videos that you access the most will stay on your device at full resolution.
To turn on Optimize [device] Storage on your device, tap Settings > iCloud > Photos or Settings > Photos & Camera, then select Optimize [device] Storage. You need an Internet connection to access an original photo or video that’s stored only in iCloud.
I am not yet using iCloud Photo Library as it is still in beta and Apple has not yet released a Photos app for it that is compatible with my Mac computer. I'm giving you the above information but advising you to proceed at your own risk.