Hello! This issue could be related to a network. I recommend reviewing our network recommendations for desk phones, which can be found at:

https://support.ringcentral.com/article-v2/Network-requirements.html?brand=RingCentral&product=RingEX&language=en_US

This resource will provide you with valuable information to help troubleshoot and optimize your desk phone's network connectivity.

If you need help, please submit a ticket at https://support.ringcentral.com/new-case.html.

sounds like she has unread voicemail messages. 
You said you deleted all the voicemail messages. 
Have you double checked the Admin portal > Users. There is a Msg. column that indicates how many voicemails a given extension has. What do you see in the Msg. column for her extension? 

You can ask RC to disable the WMI (Waiting Message indicator - also known as MWI), so that the light stops flashing even if you do get a voicemail.

And if all else fails, there isn’t anything that a bit of duct tape can’t fix. Make sure it’s black electrical tape in this case.

Follow the instructions below, it may seem like a lot, but it only takes a few seconds and will resolve the issue.

In the admin portal select the user>notifications>Messaging Notifications>change the pull down from “Basic Settings” >Advanced Settings>select Customize Notifications for voice mail>check the box for Mark message as read once emailed>done>saved.

I had the same issue and I called RC.  They had to do a factory reset on my Yealink phone.  It took some time but got it resolved in one call.