@ExEngineer
Echo can be a problem on VOIP connections, especially if the calls originate from outside of the BT network, where QOS (Quality Of Service) controls are not implemented. Data packets can arrive out of sequence.
It used to be quite common on the old BT Broadband Talk service.
If you know what network the caller is on, see if it only happens from that provider.
Business users quite often use VOIP, and call quality can be very poor.
