US communication authorities are investigating allegations by telecoms group AT&T that Google has an unfair advantage because Google Voice is not covered by federal rules that govern phone service providers.

Earlier this month, AT&T said Google is not playing by the same rules as its competitors by claiming that its Google Voice telephony management application is not a traditional phone service.

This is enabling Google to block calls to rural areas to save access costs, which competitors bound by a 2007 Federal Communication Commission (FCC) ruling are prohibited from doing.

Google Voice opponents say although the application has a web interface, it uses regular phone lines, unlike Skype and other systems that use internet data channels.

In response to the AT&T complaints, the FCC has asked Google to explain its reasons for blocking the calls, according to the Financial Times.

Google said in a blog post that it avoids making connections to local phone networks because they charge "exorbitant" termination rates for calls.

They also partner with adult sex chat lines and conference calling centres that result in "ludicrously high charges", Google lawyer Richard Whitt wrote on the company's public policy blog.

In its official complaint, AT&T called on the FCC to ensure that any new rules are applied not just to network operators, but also to providers of internet applications, content and services.

Anything less would be ineffective, legally suspect and, in all events, a direct repudiation of president Barack Obama's call for a level playing field, said Robert Quinn, a senior vice president at AT&T.