Unless your phone was just bought i believe they charge you a fee for replacement battery...
https://www.macrumors.com/2007/07/02/apple-iphone-battery-replacement-85-95/
Tough call for me since I've had it replaced in October '07. But, it's going strong, as I keep it charged and in its dock when I'm at home and charged while in the car. It's gotten down to the red one time and I immediately set up my MB and recharged it to full capacity. 😎