No Registered icon (green check mark in circle) display in the notification bar.
I use Zoiper on OnePlus One phone.
The phone run the latest version of Cyanogen OS 12.1-YOG4PAS3JL, Zooper version 1.28.
If I remember correctly this notification was working properly in Dec. 2014.
I started to notice the missing of this notification a few months ago (probably more than 6 months).
Note that I have tries to re-install Zoiper app a few times and also tries the factory default reset on the phone.