During a call you can at any point put the person on the other side on hold. This is achieved by either pressing the hold soft key or pressing the actual hold button which is labelled on the phone. To resume the call you can either press the resume soft key or the hold button again. It is handy to use this feature if you need to talk to someone else while on a call already, this will play hold music to person holding on the phone. Putting a person on hold will also allow you to answer another incoming call and switch between them.
To swap between calls if .multiple calls are open: