We’re seeing the same thing on iPhone and Android. We have a case open with Sangoma. If we get it sorted I’ll try to remember and post the solution here.
Update: we got the issue taken care of. The Sangoma Phone Desktop Client Service was disabled on our PBX. Once we enabled that Visual Voice mail worked. System Admin → Port Management
Thank you! I got it working now too!
I went into Port Management like you said, assigned an https port to Restful Apps, and enabled Sangoma Phone Desktop Client Service.