Based on Ericsson market share and the knowledge of their stack, plus the fact that some other provider use Erlang, a quick review of the market say that Ericsson has around 30 to 40% of the market of phone routers, plus another 5 to 10% from other providers that use erlang.
DianaOlympos
brightball
I seem to remember hearing it several different places. The most recent that I found was from Joe Armstrong towards the end of this post though:
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