This is in Nederlands:
http://www.kennislink.nl/publicaties/de-talenrijkdom-van-afrika
But the general point is that the area with the widest range
of languages in terms of number overall, and in terms of
phonemic diversity, are in Africa. Which should not be much
of a surprise since the location of origin for languages in
general have greater diversity due to divergence over time
and lack of sustained interaction between dialect groups.
Which leads to the secondary areas of great diversity in
phonemes and languages in general, New Guinea and
(pre-contact) Australia.
As for the Caucasus region, if you are accepting of the
standard model of modern human dispersal out of Africa,
there are two ways out: through the Sinai Peninsula, and
more likely through the Arabian Peninsula at the narrow
southern end of the Red Sea.
What we get from that is a human migration across southern
Asia and into New Guinea and Australia, which become
isolated for uncertain reasons, but also become areas of
high linguistic diversity.
The route to Europe also has two possibilities: through
Anatolia and across the Caucasus. I am not an expert here,
but I think that Anatolia had much linguistic diversity
before Greeks and Persians began fighting over it.
The Caucasus is much less hospitable to human civilization,
and may function as an isolating region, both from a general
definition of isolation from peoples outside of the region,
and within the region in a manner similar to New Guinea:
small groups of people who have difficulty traveling across
a mountainous area.
Now to get super-speculative. The Caucasus was also a region
where Homo sapiens neandertalis survived the longest and
interbred with Homo sapiens sapiens. If they had language,
some of this language may have contributed to the diversity
of languages in the Caucasus region both in terms of
phonemic diversity and general language diversity.
Again, that is highly speculative.
But overall, phonemic diversity seems to exist to the
greatest extent in regions where you'd think it would: the
area of origin of modern humans (Sub-Saharan Africa) and in
areas that were colonized early that had barriers to mixing
of groups (New Guinea, Australia, Caucasus).
[Quote-tail deleted. -BMS]