iPhone 4GS Question About Muting and Phone Calls

On my work blackberry, I have the option of being able to mute all sounds EXCEPT phone calls. I cannot find the same functionality on my (non-jailbroken) iPhone 4GS. Having this would be extremely convenient because I often keep the phone muted at work so no apps make any noise, but really do need to know when I have an incoming call.

Does anyone know if there's a way to do this? Thanks in advance!