This looks very much like my old problem, i had with voxalot, switching the adsl modem for another one, helped it for me, i'm using a sitecom modem now, also have a different ADSL(2+) provider
I did search all settings, but never found, what it could be, these could be routing problems ? or even IP "features" of certain chip sets ?
Btw. the Grandstream phone isn't the problem, i've also one connected here, and it worked o.k. when i made the modem switch...