Just my 2 cents as an ATCO:
The Squawk is far, far more important than the RT call in an emergency descent depending on airspace. We use filters to hide aircraft that aren't within a few thousand feet of our sector. Sectors are split vertically and laterally. If I'm controlling a sector from FL345 and above for example, I don't want to see turboprops on my screen, so I can filter them out.
I'll give you an example from my airspace. You're departing Europe for North America. You're passing through BAGSO to the NAT tracks At FL380 on a busy day. You need emergency descent. There's my "super" sector, an upper sector below me, Dublin upper, Dublin lower and then Dublin approach. All have various filters in place to hide nuisance flights. Your squawking 77 overrides all those filters and sets off audio alarms in both Shannon and Dublin. Everyone in every sector sees your mode c rolling like a slot machine and starts moving traffic in their sectors out of your way.
Without the squawk it takes an extra few seconds for everyone to call each other and discuss.
If you're coming off the NAT tracks eastbound with us, doing a 180 and descending is the worst bloody thing you can do. IF you can, tell us what you're doing, we'll give essential traffic if you need it, leave you alone if not. But if we see and hear that 77 alarm there's an awful lot of pairs of eyes on you and we'll get everything out of your way pronto
Last edited by Una Due Tfc; 15th Mar 2016 at 10:09.