My Zulu 3 softphone keeps ringing after I pick up the call on my mobile app or on my desk phone. Even if I close Zulu the ringing continues. I have to log back in to zulu and make a call for it to stop.
If the server and client are fully up to date then you’ll need to open a ticket with Sangoma support.
Good luck, they will tell you they do not support free products of which Zulu is for two users. It’s rather frustrating and a circle between the forum and them with no apparent eagerness to repair bugs and issues