Google Fiber is sending out invitations to an experimental telephone service for some of its high-speed Internet subscribers, according to two people who have received the invitation.

The service, known as Google Fiber Phone, closely resembles another Google product, Google Voice. That application lets users link all of their various telephones, including landline and mobile devices, to a single phone number. Fiber Phone comes with Google Voice features, such as voicemail transcriptions and automatic call screening based on the time of day.

The invite has been rolling out for at least the past month to members of the Google Fiber Trusted Tester program. The Trusted Tester program involves Fiber subscribers who have opted in to try new and potential changes to their service.

Officially, Google Fiber does not offer traditional phone service. But if the company rolls out Fiber Phone more widely, Google would become a true triple-play provider, offering customers a mix of broadband, television and telephony — just like its rivals in the cable industry.

From there, it wouldn't be much of a leap for Google to begin offering its broadband subscribers bundled access to Project Fi, Google's recently launched cellular service that hops between Sprint, T-Mobile and WiFi. As many cable companies mull an entry into the competitive wireless market, it appears Google may already be positioning itself to get ahead of them.

You can see a copy of the invitation being sent out to customers here (PDF). A Google spokesperson didn't immediately respond to a request for comment.

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