The most inexpensive adapter-based VoIP provider is..

My VoIPProvider.com's updated their listings of more than 250 U.

My VoIPProvider.com's updated their listings of more than 250 U.S. VoIP providers with a list of some adapter-based VoIP providers ranked in inverse order of yearly cost.

SunRocket's initial yearly cost of $199 ranks at the top of this compilation.

You can also see the full list here