I don't know VOIP verywell but it should like, you don't have enough upload bandwidth to send out audio. But I dont know why this would just start happening.
Do a speed test, to see how fast your upload speed is, and make sure if it fast enough for voice over ip.
Also reboot everything that is involved, router, switches ect.
Do you have anything connected to your internet that would be sending data out.
Im having the same problem, sometimes people can not hear me talk, but I can hear them. If youhave any info could you please send it over. firstname.lastname@example.org , much appreciated!!