When you’re on a call, the screen shows call options.
Put a call on hold
Tap Hold. The caller can’t hear you and you can’t hear the caller. Tap Hold again to talk again.
Use iPhone applications while you’re on a call
While on a call, press the Home 
button to go to the Home screen and use other applications. Or tap Contacts to browse contacts.
So for example, you can check your calendar or takes notes while on a call. If you’re connected to Wi-Fi 
, you can also browse the web and use other Internet applications such as Maps — to find the location of a restaurant, perhaps.
To return to the call screen from another application, tap the green bar at the top of the screen.