In my experience this problem is most often caused at the end where the speakerphone is being used.
In this phones case, considering the fact that the speaker on the back of the phone is about an inch away from the microphone it is not too suprising that people on the other end can hear themselves.
Depending on the placement of the phone and the environment you are using the speakerphone in, it may be more noticeable, ie., small rooms, small automobile, cubicle with hard walls and desk which can bounce the sound from the speaker right back to the mic.
Try turning the volume to the minimum level where you can still hear them clearly and avoid having the back of the phone pointed towards a hard, sound reflective surface if possible, for example lay the phone screen down on the table instead of screen up if possible.
If you haven't already tried this and decide to let us know how it turns out.