The feature can be interfered with by various email readers, spam filters, anti-virus blockers, etc.

To understand how this can have issues, you need to understand how the feature works. There are two ways we send voicemail emails. As an attachment or as a link.

• When the Voicemail is sent via the an "Attachment" option, within the email is an image that is not visible by the user, that image is hosted on our website and accessed via a serial number that indicates which email you are viewing. When that image is requested by your email software we mark the voicemail as played, once all voicemails are 'played' the system clears the light on the user's phone.

What can go wrong with As Attachment mode... 

• Spam / Anti-virus software may look at the image before the user even reads the email causing the light to clear right away.
• Email Clients may be in a secure or privacy mode where it will not show remotely hosted images to protect the user location. 
• When the Voicemail is sent via an "Link" option, within the email is a link to the recording, that recording is hosted on our website and accessed via a serial number that indicates which recording you want to play. When that recording is requested by the your email software we mark the voicemail as played, once all voicemails are played the system clears the light.

What can go wrong with As Link mode...

• Spam / Anti-virus software may look at the link before the user even reads the email causing the light to clear right away.

The solution varies depending on the mode and the type of email client. Generally you need to figure out what is interfering on the users end and resolve it. Using the As Link mode tends to be more trouble free, but as you see it can still have issues. Sometimes adding the domain the email comes from to a 'safe' list may help. Check with the vendor of your email client, spam and or Anti-virus software.

For instructions on how to configure exchange / outlook, see this posting from Microsoft... 

https://support.office.com/en-us/article/Change-whether-pictures-in-messages-are-automatically-downloaded-020abc02-4876-47fa-9cb1-2b4543154311#__toc306082828

*Pat# 8,260,868 B2