We swapped in one of these managed switches and lost Vonage telehony connectivity to our phones. After some debugging the problem was narrowed down to this particular DGS-1100 firmware feature:
Security->DoS Attack Prevention Settings->Blat Attack
Disabling this feature cleared up the problem, the rest of the DoS settings caused no issues for us. This problem was unexpected as we did not have this type of problem with the previous managed switch which also employed a Blat Attack feature. Though the previous switch was a different brand.
Hardware ver B2 (latest)
Firmeware ver 1.01.018 (latest)