SIp or analog or?
Echo is either acoustic or from the telephone line (hybrid, line, near, far, etc.). Echo occurs at the analog level so it is not possible for SIP, being digital, to cause echo. It would be akin to getting your computer monitor to display two pictures, which in the analog television days was possible when the signal paths of a given TV station was reflected in some manner (building, cut in a cable) which allowed the signal to be received at the same reception point but at two different times and at two different signal levels.
So, if not acoustic, then it is somewhere in the analog realm where the SIP provider or, a path along the way, is reflecting back excess energy. Echo cancelers and suppressors can be effective, but alas you will likely have to contact the provider with some details about the given calls as I doubt there is much that you can do unless, using analog lines and then you can play the TX-RX and impedance settings. This is where the impedance matching comes into play when using the FXO ports of a SIP gateway.