Also, when was it previously working ok without any echo, if ever? If it was then did anything change in the meantime? E.g. phone software update, phone settings changes, change of network provider, etc.? Can you try your SIM in another phone handset just to see if the problem persists there? (I doubt that the SIM is the issue but no harm in eliminating it was a potential issue). Does the problem happen if you use a headset (wired or Bluetooth) instead of the phone's own mic/speaker? Does it happen when you're on speaker phone?
As ever, some systematic process of elimination may help to identify the root cause...