Actually, it doesnt work… you cant monitor Group Voicemail like you think you can (the ticket i opened, Grandstream “corrected” to say it doesnt work)…
The only get around i did was create an extension (call it Voicemail or something), then list the concurrent connections to the amount of phones you need to register (i am not sure if theres a maximum).
This “extension” needs to be added to each phone as an “account”. As its a “group” generally you will push calls to it via a ring group, hat would allow to “foroward to voicemail” then select that extension.
Now that light will show up RED on every phone that has a voicemail in that box (as ANY account on a grandstream phone will also subscribe for voicemail on that account), its as easy as pressing that button to collect the voicemail, more importantly when the voicemail is deleted (or read) EVERY account like will go off once its collected.
I am not sure why Grandstream has not fixed this as its boarding on 4 years now, freePBX and Cisco work like this, Grandstream does as well, except you can subscribe to a MWI on the Group Voicemail box, Asterisk works like this as well (which is Grandstreams engine). Really have no idea why this has not been fixed.
If you need more help, call me (timezone is EST)