Unfortunately, I do not know the cause of the problem.. However I have had this same problem in the past, but with a different softphone (x-lite). Although I was never able to figure out what was causing the problem, but it seemed to cut-out at almost the same time each call.
As far as I know, The Engine VoIP service is completely compatible with all of NCH's VoIP products, so that is probably not the issue. Your internet connection is also unlikely to be the cause unless you often have connection disruptions or data packet loss.
Just for the heck of it, you can try setting up Axon with your Engine account and then set-up Express Talk to use Axon to make calls.. No guarantees that will work, but if it is a problem with Express Talk trying to talk to Engine, it could solve the problem.