ATC don't read full frequencies (e.g. 132.57 instead of 132.570 or 132.575 etc.) and often fail to check the readback to compound this;
Pilots who think it is funny to block 121.5 with idiotic broadcasts of animals and music thus, as in the UK, many pilots turn down/off 121.5 due to the distraction;
That would be 121.500 then.
(transmitted as 0ne Two One Decimal Five)