Message Center Alert

Try the info here  - https://support.microsoft.com/en-gb/help/2409256/you-can-t-connect-to-skype-for-business-online--or-certain-features-do, which expands on the link you have.  Also, there is https://support.office.com/en-gb/article/Optimizing-your-network-for-Skype-for-Business-Online-b363bdca-b00d-4150-96c3-ec7eab5a8a43.  Get the customer to confirm all the checks and guidance has been followed.

Microsoft has must have picked up on something that isn't fully working or optimized and a threshold reached to trigger that message.  Whether it's the F5 (or anything else, other firewalls etc., that are in the way) that needs double checking and did the customer end up actually change anything after going through these checks, as perhaps this message will go once Microsoft picks up on any new configuration. 

Also, more to the point, what is the Skype client experience like, is everything in working order or is anything substandard or degraded?  I'd also consider logging a service request and see if Microsoft can be specific on what needs changing if this persists.

Good luck.