Echo cancellers will have trouble working when there are too many analog-to-digital conversions along the signal path. That's why it is noticed more on calls out to the PSTN.

Sometime acoustic echo will be too much for an echo canceller to deal with. For example, someone has their cell phone volume cranked while talking to you, or their talking to on a bluetooth headset. What you would have heard as more "sidetone" before you now hear as echo, because of the inherent delay.

Brian Dunne
ETC Telecom
www.etctele.com