Echo cancel in PSTN phone

hello

i have tested four ip phones in the LAN namely soundpoint 501, GXP2000,budget tone 200 and from atcom(i guess atcom 100 something) and also PSTN phone

when i make a call from PSTN to any ip phone and vice versa i experience a little echo
too bad for ears to listen. but when a call is made thru ip phone to iphone everything is fine,
what would be the remedy on this?

i remember about RX gain and TX gain in zaptel.conf where someone told me this is responsible for the echo with PSTN phone

Any idea please

have you read all the tips on the wiki about echo cancellation ?