... Until recently, an American phone in Iran would not receive any signal. But that has quietly changed. This past week, a spokeswoman for AT&T acknowledged that the company was providing voice and data service in Iran to its customers with American phones through a partnership with a local firm, RighTel. An employee at the Iranian company, fully owned by a state entity, confirmed the partnership.

While the announcement that Airbus and Boeing will provide dozens of jetliners to Iranian carriers garnered worldwide headlines last month, the deal that AT&T clinched in March, making it the only American provider to offer phone service in Iran, flew under the radar.

The agreement is one of the few signs that the promises President Hassan Rouhani made long ago of welcoming Western businesses and ending Iran’s isolation are at last beginning to be realized.

“This is a step in the right direction,” Masoud Daneshmand, an official at the Iran Chamber of Commerce, said of AT&T’s partnership with RighTel. “The fact that phones are working in Iran and the United States is a sign of good will on both sides.”

 ... It remains unclear how AT&T and RighTel will settle accounts. A representative for AT&T said the company would not disclose information on financial arrangements made with the Treasury or with its Iranian partner. One possible clue: RighTel is owned by theSocial Security Organization of Iran, a state entity that has large stakes in several domestic banks.

The Treasury would also not speak about the deal, saying in a statement that it “generally does not comment on specific licenses or engagement with private parties.”