YouMail uses conditional call forwarding to ensure that your calls reach YouMail. Once the caller reaches YouMail they have the option of hanging up or leaving a voicemail. YouMail keeps track of both hang ups and voicemail for you, but if the caller hangs up too early YouMail will not be able to track the hang up.
This is occurs because YouMail must gather the Original Call Number (OCN) information before the caller hangs up. This information includes the phone number, area code, location, and name of the caller. If the caller hangs up before YouMail has the opportunity to receive the OCN information, the hang up will not be tracked by YouMail.
Note: that if you are receiving voicemails, but not hang ups it may be because the caller is hanging up before YouMail has the chance to gather the information. This does not mean your call forwarding or YouMail are not activated, it is simply on the callers end. You can always test call your phone from another phone to make sure it rings like normal before forwarding to YouMail's voicemail.