<div>Two things come to mind. First, (I'm not familiar with the SonicWall, so this may be way off), could it have suddently decided that your Voip provider's IP address is a threat? From what I understand, Cisco uses some technology such as this. If it thinks there is a threat, it starts blocking things. Maybe try rebooting the firewall. I read that somewhere in the Cisco mountain of documents that it doesn't store this information permanently, just in RAM, and it gets reset upon reboot.
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<div>Second, could your ISP have started blocking Voip packets? I imagine some providers who sell Voip services are going to start blocking and/or dropping packets.</div>
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<div>I can't be sure of this, but I never was able to have a Voip call last more than about 1 minute on Time Warner cable. My whole connection would drop. Switching the cat5 cable from Time Warner to my DSL provider (all other things being the same), I get calls that last as long as I want.
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<div>Could be something to check into. If nothing changed on your end, it sounds like something changed between you and your Voip provider. Getting your ISP to admit to it, could be somewhat of a challenge.</div>