What causes Telephone Echo?

Echo is experienced in normal PSTN connections and also in VoIP connections, but due to the inherent delay in VoIP, it can actually be more noticeable.  Echo that a caller hears is probably the result of the far-end, where an impedance mismatch bounces signals back from where they came, and in today’s global telecoms networks calls can transit a mixture of IP and PSTN network, on copper, fiber-optic, and wireless transmission legs.  Due to notable delays, there are echoes of one's own words audibly discernable. 

Research indicates that when there is a delay of 35ms or less in one-way transmission (round-trip delay of <70ms), the speaker cannot distinguish the echo from an acceptable level of sidetone. However, as one-way delay increases beyond 35ms, the echo grows more problematic. The longer the delay, the more dramatic and invasive the echo becomes.