I'm curious if you've had other VOIP providers with the same ISP and whether they've been flawless? Care to mention the ISP and level of service you have?
I currently using VOIPo (primary) and Sipgate (alternative). The VOIPo is Linksys RT31P2 and Sipgate is Linksys PAP2-NA (unlocked). Both of them are behind my routers (Linksys WRT54GS). I have no any problems with both the services.
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