That is more a function of the phone. SIP to SIP calls do not have echo cancellation. The only echo that would be generated is acoustic and you are dependent on acoustic echo cancellation done by the phones.
Some of the variables in that equation are the hand/head sets used and the amount of delay traversing the network. Bad headsets can generate excessive acoustic echo that the phone(softphone) echo cancellation cannot handle. Excessive delay could also fall outside the range of the echo canceller.
So assuming you are using half decent headsets and do not have excessive delay I would ensure the softphone has echo cancellation enabled and if possible try make some adjustments there or try a different softphone with better echo cancellation. I don’t know anything about SJphone so I don’t know how well the echo cancellation works.