As I’ve written about before, I use a combination of Vodafone 3G and the AT&T Network Client on Windows to access my corporate VPN. Recently, I’ve been seeing this error a lot when ‘dialling’ the 3G network (I connect it via the Windows dialler, rather than the Vodafone client):

This morning, I figured out what it was that was causing the problem. AT&T was open when I was dialling, and had the modem selected in the ‘Cellular’ tab (which I don’t normally use):
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