1. When you're on a call, the screen shows several call options.
   
2. To mute your line, tap mute. 
   Note: To unmute, while muted tap mute. 
   
3. To put the call on Hold, hold the mute button until it turns into the hold icon (about 5 seconds).
   Note: To take the call off hold tap hold.
   
4. To turn speakerphone on and off, tap speaker. If using a Bluetooth device, tap audio, then select the desired option; Bluetooth device, iPhone, or Speaker.
   
5. Tap contact, to open your contacts.
   
6. Tap FaceTime to make a FaceTime call.
   
7. Tap add call, to make another call.
   
8. To end the call, tap the End call icon or press the Sleep/Wake button.
   Note: Learn more from Apple support article: iPhone: Understanding phone features