When calling via VoIP the system set's the disposition of each call according to what happened. If the number is busy, the disposition is "Busy" and so on. But one thing the system can't easily determine, is whether the call reached a voicemail, or if it was a real conversation. To that end, we need the agents to help us. That's why there are two hangup buttons, one for when the call was a normal call with a person on the other end, and one for when it was a voicemail. Selecting the correct button to hang up the call is important if you want to get a few key metrics right - The accuracy of these metrics is dependent on agents choosing the Voicemail disposition when they connect to voicemail.
Contact Rate - The percentage of unique users that answered calls compared with unique users that were called. It's one of the traditional call centre metrics and it's indicative of the quality of data.
Hit Rate - The percentage of successful (won) calls compared with unique users that answered calls. It's also one of the traditional call centre metrics and it's indicative of agent performance.
To register the Voicemail disposition in the system, agents should either use the voicemail hangup button in the dialler or select the "Voicemail" category when actioning a lead (the latter requires the call to be connected when selecting the action/category).
So to recap - You can set the call disposition to "voicemail" in two ways:
By selecting a category called "Voicemail" from one of the action button dropdowns while the call is still connected.
By selecting the voicemail disposition button in the dialler - if you have voicemail drop enabled a drop-up will appear and then selecting any option will work (either a voicemail drop or just hanging up).
If you select the voicemail category after the call has ended, it will not disposition the call as voicemail (but it will still record the category in the activity log and in reports).