AT&T, the largest US telecommunications group, called on federal regulators to force Google, the internet search and advertising giant, to “play by the same rules as its competitors” and ensure that its Google Voice application does not block calls to some rural areas.

The call, which came in a strongly worded letter from AT&T to the Federal Communications Commission’s competition bureau was signed by Robert Quinn, AT&T’s senior vice president for federal regulations. It marks a significant escalation in the dispute over Google Voice and proposed new rules on “net-neutrality.”