Bzzt! Wrong. Sadly you can’t just apply this rule to other -ix words. Consult your dictionary if in doubt. Then, if you disagree, insult it.
And BlackPheonix (sic), I bet you wish you consulted your dictionary before registering that name 
By the way, my (British) dictionary says that the accepted US spelling is “phenix”, so it’s kind of weird that the city is spelt the British way.