I use Vonage for phone of a solid Fios internet connection and the Dish modems have no problem. I believe, however, that I have read posts from VOIP users having problems that were solved using a suffix on the dialed number or similar... you may find something with search.
Some VOIP services do need to have a prefix in order to dial out properly. For example, Vonage used (I think it still uses) *99 as a prefix