Purchasing more iCloud storage doesn't increase the capacity of your phone. The capacity of your phone was fixed when you bought it. The additional iCloud storage only gives you more room in your iCloud account for data you sync with the account and your iCloud backups.
To see how much free space is on your phone, go to Settings>General>Usage. It will say how many GBs of free space are left. If you don't have enough to store all your music, you'll either have to store less music on your phone, delete other data from your phone to make more room, or subscribe to iTunes Match (https://www.apple.com/itunes/itunes-match/) to enable you to access your music library from iCloud.
If you purchased the extra iCloud space by mistake, you can request a refund within 15 days by calling 1-800-MYAPPLE (if you're in the U.S.).