Why are the battery life not draining as fast as when on a call when using ipod ?

[iMore Question]

See when the phone is used to take calls the battery drains real fast. But when listening to songs on iPod on the same phone, the battery doesn't drain that fast. I discovered this myself because when I walk daily I listen to music continuously for 45 minutes and it doesn't drain battery as compared to how it drains when on calls.