Android app rings when offline

Question

I have Zoiper android app registered with a 2talk.co.NZ sip service which works fine. However when I use the widget to go offline and the main app shows the service is "not registered" it still rings on incoming calls and I can make outgoing calls.

Is this a fault? How do I easily go offline? Why does the phone still ring and make calls when unregistered?

Thanks for any help.

Android 5.1.1 Zoiper 1.38