There are two out there that I know of. One I from time to time do intergration for called
Cbeyond.net – Small to mid size companys, includes full T1, for Voice Data, Fax and 2000 mins LD for 525.00 a month. They use the CISCO IAD router. The other is vonage.com mainly for res and SOHO users. Which I use. Cost is 41.19 per month with umlimited LD. The service uses a cisco ata186 and really works great. I use about 5000 mins a month so that really cuts my long distance bills big time. Call qaulity is better 95% of the time than my land line. The do require a broadband connection though, DSL,Cable,T1, ISDN. As far as custmore service both are top notch.
Take a look at them you might be interested. You can also checkout a forum at dslreports.com under