Make Calls From Google Voice App For iPhone :

The easiest way to place a call that displays your Google Voice phone number is by tapping the Quick Dial tab and selecting one of your favorites or a recent call.

Make a call:

1. Open Google Voice on iPhone.
2. Touch the Dialer tab.
3. Enter any phone number (1-xxx-xxx-xxxx for US numbers and +cc xxxxxx for international numbers) and touch Call.

Check the credit balance in your Google Voice account:

1. Open Google Voice on your iPhone.
2. Go to Settings.
3. Select Balance.

For all calls that are not free, you should hear a message notifying you of the call rate. If you don't hear the message, your call is not going through Google Voice and your call may be subject to your carrier's rates.

Via : https://support.google.com/voice/answer/1065740?hl=en