At 5 minute intervals during a call engin’s servers will send a re-invite. If it doesn’t get a reply within 32 seconds, it terminates the session. This is done so if you drop your internet connection, you don’t end up with calls staying open and you still getting billed for them. Other VSPs will do similar things at different time intervals e.g. Nodephone drop at ~11 minutes. Engin probably have one of the smallest re-invite intervals and hence to the uneducated, it could appear to only happen to engin and not other providers.
It is generally always a NAT issue in local hardware (router). If the re-invite request is not transversing your NAT router and reaching your ATA, your ATA never has a chance to respond, hence the call is terminated at 5:32.
More notes on Whirlpool FAQ