The Echo that happens is mostly the carrier's fault (Verizon, T-Mobile, AT&T, etc.) It doesn't matter if on cable, FIOS, or cell tower. Weather (rain) can also play into account for Echo. Even a bad cell tower can cause this.
You can also report this to customer service to have a tech check the cell tower if it happens at the exact location.
Note-Also, it can occur when the receiver of calls uses WiFi/Cell/computer to answer calls or make calls remotely being plugged into the router using the desktop or laptop.
It's infrequent for the mic on the phone to fail unless it's dropped from a high height.
Your best bet is for the receiver and caller to reboot and try again. And if that doesn't work, wait until you get strong signal strength and reboot.