That area has radar control – Amazonico Center - after implementation of SIVAM and it’s been better than some years ago… BUT, I’m not sure if radar coverage AND two-way VHF radio communication have been in service without restrictions. It is not unusual to find out some problems with both. After flying for those routes several times in my life I get used to hear the same controller’s voice on different frequencies including airplanes flying on the same route using different frequencies as well. And also communication in English can be very difficult under situation beyond routine.
The worst to date occurred in 1982 when a Vasp 747 crashed…
In fact it was a B727-200.