There's a thing in phone circuits (part of your call goes over wires, even cellphone to cellphone, and part of it is nalog) called an echo canceler. The cause of almost all echo in phone calls is a bad echo canceler somewhere in the call's path (or some impedance imbalance that causes the echo canceler to not work well). Call your network operator, talk to tech support and ask them to escalate the trouble to level 2. Someone is going to have to call an engineering department to locate the trouble and fix it. (It usually doesn't last as long as yours has because someone reports it almost immediately.) (Or call Sprint directly, or go to a Sprint store, tell them the situation, and see if there's someone at the store who knows enough about how things really work to know who to call at network operations.)
The fix usually takes bout 2 minutes. Finding where the trouble is, and getting to it, could take days. (In olden times, when "computers" were things inside the heads of human beings, it sometimes took waiting for a circuit to go totally dead before the problem could be found. Then there's he Verizon order line that ... well, don't ask, but about 8 million people couldn't call any phone not in their own central office for almost an entire day - because of 1 broken wire. It took so long because Verizon had to call from one central office to another, nd ... well, that was the problem they were trying to fix. Kind of pulling yourself up by your bootstraps when you left your boots at home. Awkward, to say the least.)
