On a basic flip phone, if you are on a call, and get another, press the SEND or ANSWER button. That puts the first call on hold so you can answer the incoming call. When done, press SEND again to go back to the first call.
At least, that's the way it's supposed to work...I remember trying and it seemed like all I did was hang up on everyone. Now, if I get an incoming call while I'm already on the phone, I just ignore it and call back when I'm done.