I find that the 'Quiet Area' in the Flounge near the former Champagne bar is, generally, quite quiet. Not everyone observes the sign prohibiting calls but most do, and if anyone didn't I would point to the sign.

Of course - if you have access - the Quiet Room in the CCR is a real haven of tranquility. I once spent nearly 8 hours in there and only had 1 other person briefly come in.