A few things you can try to narrow in on the cause of the problem....
Turn your volume down. The sound from the speaker in the earpiece sometimes makes it to the microphone thru the internals of the phone. Turning the volume down will tell you if this is the problem. If it is, this is a possible fix for it....http://youtu.be/FkcxzmCPFWw
Take the case off. Sometimes the sound from the speaker in the earpiece travels thru the case and reaches the microphone.
Turn off noise cancellation.
Keep track of when it does and does not happen (after reboot, certain callers, 3g vs 4g, cell tower...)
Echo should not happen. Sometimes it is a physical problem with the phone. Sometimes it is a bug in the programming of the phone. If you can't resolve it and you wan't out of your contract, write a letter to the FCC. That quickly got me out of mine with T-mobile and 3 phones (all samsung S3) that had echo problems.
I had a friend on T-Mobile with an older S3 with the echo problem. I fixed it using this video....http://youtu.be/FkcxzmCPFWw