<<turn UP 121.5 as that may be the only available frequency for ATC to get through to you on due frequency congestion?>>
With the usual proviso that I've been retired a while and procedures may have changed... During my 31 years in UK ATC I was never trained to, and never trained anyone, to even think of using 121.5 to contact an aircraft when the normal frequency was busy, or for any other reason. In the event of a radio failure we would ask the company if they had contact, but never ever use 121.5. 121.5 is an EMERGENCY frequency, plain and simple.