The echo that the remote called hears, might be of either electrical or acoustic origin.
If electrical, it would indicate a mismatch between the line impedance of the Dahdi card or device and that of the phone extensions being used. You might be able to adjust jumpers or settings on the Dahdi card to select an impedance which matches the phone better, and reduce the echo. Or, use a different type of phone, having a better-matched hybrid in it.
If acoustic, it would probably be due to bad phone handsets. I’ve seen quite a few cheap phones where sound coming out of the speaker could travel through the handset body (like going through a pipe) and reach the back side of the microphone and be picked up by the mike. Better phone handsets don’t have this problem; those that do can sometimes be improved by stuffing the inside of the handset with some sort of damping material such as cotton wading or Dacron pillow stuffing.
Speakerphones are, of course, notorious for generating echo.
SIP phones won’t have the impedance-mismatch issue at all, and their handsets may simply be better-designed with respect to acoustic feedback and leakage problems.