As the country upgrades its old, copper telephone lines to newer technology, the companies that operate those networks face a lot of unforeseen obstacles. The process is supposed to be complete by the later part of the decade and could enable new features in telephony such as HD voice calls and improved 911 service. But it could also bring a range of consequences that nobody can really predict.

That's why telecom regulators have given carriers permission to start experimenting on a limited basis with a fiber-optic rollout. The transition to fiber-based networks — which treat phone calls as packets traveling atop the Internet — is being done in select markets of each phone company's choice.

AT&T has now begun planning for its own trials. In a filing to the Federal Communications Commission, AT&T lays out its three-stage plan for how it will select its test sites.

"AT&T intends to select locations for the service-based experiments that represent the challenges it will face with the transition," AT&T's Frank Simone wrote in a letter to the FCC, "and conduct an extensive review of the services to be impacted as part of this experiment."

AT&T claims it won't simply test out the transition in places that are easy, but it remains to be seen which locations it has in mind. I've reached out to the company for more, and I'll update if and when I hear back.

The trials represent an important indicator for the FCC as it decides how to regulate the new Internet Protocol-based phone networks. There's no clear point at which the agency is supposed to intervene in the years-long process; it will do so only if it sees something go wrong. Where to draw the line between a minor problem and a major one that calls for government scrutiny is among the questions this whole process is intended to sort out.