What kind of router do you have the Vonage device connected behind? The periodic loss of quality at regular intervals sounds like a SPI firewall that cannot keep up with the traffic flow. The other question is why didn't you at least try putting the Vonage device first? It would be interesting to find out if the problem goes away.
If you read the main VoipTechChat forum, you would see that router issues are quite common.
And I'd like to express my hope that AT&T does not go on strike in the next 10-15 days, in which case your U-Verse installation (and your porting) may be rather a mess.
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