you guys are spot on.last night i heard a very interesting scenario where a 737 had been handed over to area,who immediately told him to switch back to radar as there was head-on opposite traffic at his level...he duly did this and after about three seconds checked in on frequency with his callsign.immediately after he called in,he was squashed by a springbok who wanted descent or something,then an expressways.It took about ten seconds for him to finally get through and resolve the conflicting traffic situation.