said by battleop 
:Not a very good one. You could have a primary / secondary setup where you might have sipserver.primarydomain.com and sipserver.secondarydomain.net and have your domains registered at two different places. Your equipment would have to support this and you would have to use DNS servers with different domains with different registrars.
That's all I've been able to come up with also. I was thinking more related to web infrastructure, so in my case we'd have to somehow notify our clients to use a different FQDN. Not exactly ideal or really even feasible at all.
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