Receiving multiple email messages with subject line "Account is unlinked"

you can read about account linking (aka account federation) here:
https://support.ringcentral.com/article-v2/9415.html?brand=RC_US&product=RingEX&language=en_US

for example if you have an international business, you can break it up by regions and have a different account for each region. Then link the accounts together, so that  the users see each other in the directory.

When you are getting a message that an unknown phone number is unlinked, I think that’s probably the company main number for some other account that is unlinked from the federation.