There are two buttons. An obvious (to me!) one that releases the vertical clip and a 2nd more recessed one next to it to do the horizontal.
Have a good feel along the bottom (oo, er) of the clip and you should locate the 2nd one.
Hey. Thanks to both of yous I have now done it. Yeh. With not seeing underneath the cradle I didn't know about 2 buttons. I did what you said and just kept pressing along underneath and sides finally expanded. I have not used it from the start I just thought my mobile must have been to big. Great!!