Can you offer more info:
What model iphone?
Does it charge to 100% after being charged?
How quickly does it run out of power?
More info could help here.
Also, the batteries in the phones are rated for a certain number of recharges.
Perhaps your phone's battery has reached it recharge limit.
If possible, purchase a new battery through Amazon or eBay. DO NOT trust Verizon when they tell you that a new battery will cost the same as a new phone, or something to that effect. I listened to them and upgraded my phone as a result to one that I absolutely hate.
I don't know about smartphones, but basic phones, you can get batteries off of Amazon or eBay for a few dollars.