In Call - unable to view other apps!

Previously, whilst in a call I could go to other apps and get information, and then go back to the call. Despite still being in the call when I look on my desktop there is nothing to show I am still in that call. If I then need to press any numbers, eg for a selection within a company, it assumes that this is a new call and overwrites my current call (which doesn't recognise that I have made a selection - help!