While in an active call, tap the Hold icon 

 

The call will then be placed on hold and the other user will hear hold music (if enabled)

 

To resume a held call, simply tap the Resume button 

 

To transfer a call that is on hold, tap on the Transfer button

 

You will then be prompted to input the transfer destination. You can search for a user in your organization and tap on the result

 

Then choose the transfer type

 

Or you can enter a number and tap the number from below the search bar

 

And select the transfer type