DNS based geographic server load balancing (GSLB) just fundamentally does not work when you need to provide any sort of QOS on the service. Why?

What happens when your customer is in Los Angeles, but their ISP’s DNS server is in Texas?

That’s right, you think that your customer is in Texas and send them to a server in Atlanta even though your Los Angeles server is closer. If you are going to use DNS based GSLB for a service that is finicky (i.e. VoIP), give your customer the tools to select their own mirror.