Hello and welcome to the forum!
If you double press the middle button whilst on the call it will take you back to the main homescreen and you can access anything you like there. You will also see a grey dialer at the bottom of the touch screen which allows you to drag up and access the dialer if you have to press option 1 or 2 etc to get through automated call centres.
The left hand Menu button also gives you access to certain functions whilst in call.
If you want to get the call screen back having accessed your homescreen, drag down the notification bar and tap on the call. You can also quick end the call by the on/off button on top.
Think I have covered it all!! But if you do have any other queries, please feel free to ask.