You interrupt your call to conduct an enquiry with a user (including external users) and then resume the original call. 

You are on a call. 

Activate enquiry. The current call is placed "on-hold", so that the first user waits. 

Enter the phone number for the enquiry call. 

The number is dialled. The user answers. The enquiry starts. 

The enquiry is ended and you return to the call "on-hold"

The second user hangs up. You are reconnected to the first user. 

Call the system menu. 

Select and confirm the menu item. 

You are reconnected to the first partner. 

However, you can also 

toggle between the partners (),

set up a conference () or

transfer the waiting partner to the second partner ().

The second user is busy or does not answer

Cancel the callback. You are reconnected to the first partner. 

During the enquiry you can 

book a callback (),

activate call waiting () or

override().