Hang up on voicemail - voicemail calls back and continues

I know this is normal, say if if I flash to another line, then hang out, and the previous line calls back.

But this happens when I call my voicemail, finish what I’m doing and then hang up. The voicemail calls back and rings until I pick up, sometimes it happens, sometimes it doesn’t. Sometimes it calls back only once, sometimes it calls back many times.

Any ideas?
I’m running Asterisk on Debian Sarge and I’m using GrandStream HandyTone 268 units attached to standard telephones.